The reference lists are to help possible traders. I only own exclusives for the PS3, and I own a lot of them. I need to put that list there, otherwise, I get offered games I already own by everybody. This way, people can look at the list, and then not offer me games I already own and waste our time.
And quit acting like this is the only thing that makes the search engine useless. The worst offenders are the people who post "CML for xxx". Then it appears in the thread, but never gets deleted. I say get rid of those guys before getting rid of the reference lists.
But I will have to agree that keeping a list of stuff already sold is stupid. That serves NO purpose.