I think the comparison thing is a good idea but for me the sell/wish lists would be top priority out of the two ides you gave. As far as I can tell you can't casually browse these two lists, meaning that unless you specifically check every individual Japanese Saturn game in the database (just to use a format I'm personally interested in) you can't tell what other users are offering. Wish list appears (again, as far as I can tell) to suffer the same thing - unless I go through and check every game, I can't tell if somebody wants a game I'm trying to sell.
On a tangent some other stats might be interesting to see - top ten collectors for each format or something like that, just so you can easily go and have a nose through
