You can also go to a cards page and under the price click on "mentions in collections" then go down the page and click on "For sale/ trade" and you will find a list of people that have that card up for trade. Click on a name and see if you can make a trade with them. I use this a lot. But I center click on the name so that person page opens in a new tab and I can keep the list open on its tab, so I can go back quickly to other names.
I make up a lot of my trades this way. I pick a set I need cards on, see who has what and make trade proposals up and send them. It's a great way to work on a set when you are under 10 cards needed.
What is that behind you!?! Oh, It's me! Looking at the cards you have, That I want. :)