Are there any products/services that let games developers provide real-time trading and/or auction services to their users/players.
E.g.
Let the developer define what assets can be traded and the end-user application interacts with the service. I'm looking for anything from platforms as complex as real-life trading systems through to auction services in games like World of Warcraft.
I can't think of one. I doubt such a system would be very practical at this point in time for game applications due to the strict requirements for the transactions to prevent object duplication bugs. Any system that was both generic and safe would probably be more complex than writing a bespoke system for your own game (since you can make simplifying assumptions about your own game data and message passing systems that a generic system could not).