  Shill bidding occurs when fake bids are introduced into an auction on the
seller's behalf in order to artificially inflate the final price. This is
typically achieved by the seller having friends bid in her auctions, or the
seller controls multiple fake bidder accounts that are used for the sole
purpose of shill bidding. We previously proposed a reputation system referred
to as the Shill Score that indicates how likely a bidder is to be engaging in
price inflating behaviour with regard to a specific seller's auctions. A
potential bidder can observe the other bidders' Shill Scores, and if they are
high, the bidder can elect not to participate as there is some evidence that
shill bidding occurs in the seller's auctions. However, if a seller is in
collusion with other sellers, or controls multiple seller accounts, she can
spread the risk between the various sellers and can reduce suspicion on the
shill bidder. Collusive seller behaviour impacts one of the characteristics of
shill bidding the Shill Score is examining, therefore collusive behaviour can
reduce a bidder's Shill Score. This paper extends the Shill Score to detect
shill bidding where multiple sellers are working in collusion with each other.
We propose an algorithm that provides evidence of whether groups of sellers are
colluding. Based on how tight the association is between the sellers and the
level of apparent shill bidding is occurring in the auctions, each
participating bidder's Shill Score is adjusted appropriately to remove any
advantages from seller collusion. Performance has been tested using simulated
auction data and experimental results are presented.
