If you click on a player's page, then "collection summary", then scroll down to Reports, you can get a list of "Members with cards from this person on their For Sale / Trade list", which is sorted alphabetically by username with the number of cards they have of that player in parentheses. If you click on that username, you get taken to that user's For Sale/For Trade list narrowed down to just cards for that player. I didn't actually know if this feature existed either, but it's gonna make my life a lot easier too.
New collector. College student so I'm sometimes slow in replying to transaction proposals. Go Twins!