Are you planning on making it trade friendly? I just had this idea that I think would be pretty cool (perhaps You have already thought of it).
Put in an option for a wish list of collectables, so that users can select which collectables they are interested in obtaining. Then when somebody wants to sell one of their collectables they can mark it for sale and see a list of who is looking to buy it. From there, perhaps reach out to them with a price. Or maybe the other way around, when it's marked for sale, the interested people get notified and they can contact with their offer. Or in an attempt to be a little fair and try to avoid low balling or other such issues, perhaps the seller can list a price they are willing to sell at and the potential buyers are alerted of that price and whomever is willing can then buy it. Anyways there are quite a few ways to implement, but that's getting into specifics. At this point I'm sure you're just looking at general items for the site/database. So the general would be the want and sell options, and the specifics would be how those people interact and complete a transaction/trade.
