I would have a suggustion that I would like to see in the advance search.
About Search Games Owned, if you click the box for Specify User, the search will include all games the user has which are part of your search.
Why not go a little further than that.
For an example, if a gamer has many wargames in his collection and wanted add more to his collection, he would do something like to following:
Click on the + sign beside wargames, and click an option which would not include games which he does already owns in his search. The main reason for it would be to make the search list shorter, more specific and helpful.
You don't need to see game names which you already own in some cases.