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As discussed above, the design of a competing double auction marketplace consists of the design of market policies and a charging strategy. Intuitively, we can see that the effectiveness of market policies and charging strategy is affected by the traders’ behaviour, which is determined by their strategies. In more detail, in the context of multiple competing marketplaces, traders need to decide which marketplaces to participate in (determined by their market selection strategies) and how to submit their shouts in the selected marketplaces (determined by their bidding strategies).

Thus firstly, we need to obtain a fundamental understanding of traders’ strategies in terms of market selection and bidding. Based on this, we can analyse how marketplaces should set their market policies and charging strategies to make profits while still maintaining traders.

In more detail, in the context of multiple competing double auction marketplaces, traders’ mar-ket selection and bidding strategies not only depend on marmar-ket policies and the charging strate-gies, but also depend on other traders’ strategies. Specifically, there exists a positive size effect (Ellison et al., 2004), whereby, buyers(sellers) prefer marketplaces which have a larger number of sellers(buyers) since this gives the buyers(sellers) access to more choices. Such an effect will always push all traders towards concentrating into a single marketplace. However, in addition to the positive size effect, in double auctions, buyers(sellers) also compete with each other in order to be matched with sellers(buyers). This is referred to as a negative size effect (Ellison et al., 2004), whereby, traders prefer marketplaces with fewer other traders on the same side.

Thus this negative size effect will push traders to distribute across different marketplaces. The positive and negative size effects have contrary impacts on the traders’ distribution across multi-ple marketplaces, and so enhances the commulti-plexity of analysing traders’ strategies (especially for market selection). Furthermore, we are interested in analysing which effect has a larger impact, and whether competing marketplaces can co-exist, and the competition can be maintained, or whether the marketplaces collapse to a monopoly setting where all traders move to one market-place. This is important since competition drives efficiency and offers more and better choices to traders. Moreover, in contrast to much existing work that makes simplifying assumptions that all traders are homogeneous with the same preferences (i.e. types), or that marketplaces have com-plete information about the types of traders, many realistic marketplaces have heterogeneous

traders whose types are only privately known. Specifically, in such settings, trader types can be drawn from a discrete distribution (i.e. discrete trader types), or can be drawn from a continuous distribution (i.e. continuous trader types). These assumptions also increase the complexity of the analysis.

Furthermore, in the real world, there usually exist three types of trading environments. The first is single-home trading where both buyers and sellers can only select one double auction marketplace at a time. The second is multi-home trading where both buyers and sellers can participate in multiple marketplaces at a time. The last is hybrid trading where one side of traders can only enter one marketplace at a time (i.e. single-home trading), while the other side of traders can enter multiple marketplaces at a time (i.e. multi-home trading). Different trading environments will affect traders’ strategies, and, in turn, affect the effectiveness of the market policies and the charging strategy. For example, in a single-home trading environment, traders will only participate in the most profitable marketplace, and thus marketplaces have to compete fiercely with each other to attract traders, and have to charge low fees. However, with multi-home trading, traders will participate in any marketplace that provides non-negative (or positive) profits for them, and thus marketplaces can charge higher fees to maximise their profits. In addition to the impact of the trading environments on the marketplace competition, the properties of the goods traded between buyers and sellers can also affect the competition.

Specifically, when multiple goods are traded across multiple marketplaces, these goods can be either independent, substitutes or complementary. When they are independent, the trader’s value for the multiple goods is additive, i.e. equal to the sum of its value on each individual good.

When they are substitutes, the trader’s value is subadditive, i.e. less than the sum of its value on each individual good. When the goods are complementary, the trader’s value is superadditive, i.e. greater than the sum of its value on each individual good. These different properties also affect traders’ strategies. As an example, when trading complementary goods, buyers may prefer to buy as many goods as they can, and thus will try to bid high in several marketplaces to make more transactions. Therefore, when analysing competing double auction marketplaces, we need to consider these different situations.

In this thesis, we consider all the above factors when analysing competing double auction mar-ketplaces. More specifically, the research challenges of this thesis that deal with a number of issues in the competing marketplace design are as follows:

1. Analyse the traders’ market selection strategies: In the competing marketplace context, traders can move freely between marketplaces to search for the most profitable one. In-tuitively, we can see that how traders select marketplaces is important since this will sig-nificantly affect the competition result of marketplaces. For example, this will affect the traders’ distribution across marketplaces, and, in turn, affect the market profits. Further-more, this will also affect the marketplaces’ decisions of establishing market policies and charging strategies. Thus firstly, we should obtain a fundamental understanding about traders’ market selection strategies. In the double auction with multiple buyers and mul-tiple sellers, the optimal choice for a trader in terms of selecting a marketplace not only

depends on market policies and fees charged to it, but also depends on the behaviour of other traders. Thus we need to analyse the equilibrium market selection strategies for traders. Furthermore, as discussed above, traders’ market selection strategies are affected by different trading environments and different properties of trading goods, and thus we need to consider these factors in the analysis.

2. Analyse the traders’ bidding strategies: After selecting marketplaces, traders need to de-termine the shouts which they should submit in the chosen marketplaces. How traders submit shouts is also important since it will also affect the marketplaces’ decisions of setting market policies and fees. Thus in addition to the market selection strategies, we also need to analyse the bidding strategies for traders. Similar to the market selection strategies, in double auctions, the optimal choice for a trader in terms of placing a certain shout not only depends on market policies and fees charged to it, but also depends on the shouts placed by other traders. Thus we need to analyse the equilibrium bidding strategies for traders. Currently, most work on bidding strategies is restricted to single marketplaces without considering inter-marketplace competition. Actually, even for the single market-place, the equilibrium bidding strategy for traders is a challenging problem. In addition, in the competing marketplace context, we also need to analyse traders’ equilibrium bidding strategies across multiple marketplaces. Furthermore, traders’ bidding strategies are af-fected by the different trading environments and the different properties of trading goods as well, and thus we have to incorporate these factors in the analysis.

3. Analyse the market policies: The first part of competing market design is the choice of the market policies. A considerable body of work exists on the market policy design of an isolated double auction marketplace without inter-marketplace competition (in section 2.3.2.2, we introduce this work in detail). However, we do not know which market policy will perform well when competing with other policies. Currently, there is no systematic work on analysing the performance of market policies in the competing environment.

Furthermore, the effectiveness of a market policy also depends on the traders’ behaviour.

Therefore, we need to analyse how different market policies affect the performance of competing marketplaces by considering different behaviours for traders.

4. Analyse the charging strategy: The second part of competing market design is the choice of a charging strategy. By charging fees to traders, the marketplaces can make profits.

However, as mentioned previously, there exists a conflict between attracting traders and making profits. Thus the competing marketplace has to be able to set appropriate fees to maximise its profit, while at the same time maintain the number of traders at a good level.

Furthermore, during the competition, the marketplace’s opponents may change their fees and traders’ bidding and market selection behaviour may change as well, and thus the marketplaces should be able to adapt their fees to these changes. Moreover, in the real world, marketplaces charge different types of fees, which usually have different effects on the traders’ strategies and on obtaining profits for marketplaces. For example, when an entry fee is charged to traders for joining a marketplace, some traders may not choose

the marketplace because of the potential of negative trader profits that may be caused by this fee. Thus charging an entry fee may result in good market profit for the marketplace, but cause a decrease in the number of traders entering the marketplace. Similarly, when a percentage fee is charged on transaction profit made by traders, traders will choose this marketplace since such a fee guarantees non-negative profits for them. However, the marketplace may not be able to obtain a good market profit even by charging a high percentage fee since traders can shade their shouts to hide their actual transaction profits (as we will show in Section 4.4.1). Therefore, we need to analyse what types of fees are appropriate and effective for marketplaces to obtain market profits and maintain traders.

5. Design and evaluate a competing marketplace: Once we have analysed market policies and charging strategies and obtained insights from the analysis, we will use these insights to design a practical competing marketplace agent. Furthermore, in order to test the ef-fectiveness of our design, we will evaluate it in the CAT competition, an international benchmarking exercise in this area.