I think the PM/IM along with a trading system works best when the adoptables site co-functions with the forums (for example like Pokeplushies, where logging into the forums also logs you into the actual adoptables site). Of course, they have loads of different ways to give your adoptables to other users: for free like an abandoning area, for a price of points that the user sets, and for a trade that the user must accept.
For a real trading experience, something like a street vendor that keeps exotic pets and sells them for points (after all, we gotta put that point system to use somehow) would be something I'd like to see. He or she could keep some in special cages that only a specific user can open (which is preset by the seller) and would be a very little to no cost.
Just throwing out ideas.
