If you go to your collection summary page and scroll down you can see links for members with cards on their want lists and members with cards for trade. What I do when I look to finish a set is I'll click on the members with the most cards on their for trade list from that set and start clicking profiles and looking for matches. Then when I get down to just a few cards I'll click on those specific cards and click the green number to see who has it up for trade and look for members with matches.
That's also why I tend to add a bunch of other cards to my want list that I only partially care for. I might get a trade with only 2 or 3 cards from a specific set I want but I'll be able to put together a 20 card trade because of other matches from the so so cards on my list. Makes it worth the postage and I end up finishing off sets I started that I'm not always actively looking at.