There are plenty of people that will trade with you. Go find the puzzle you like and just start the trade. To trade multiple, you start multiple individual trades. When you ship them, pack them all together in one box or wrap them together in one paper wrapper. When you get the tracking number, put that same tracking info on each of the trades you started. Having the same tracking number is what bonds all the trades together. When you mark one of the puzzles as ARRIVED, the others with the same tracking number also get marked.
For help on shipping multiple packages, click the menu link in the top/right corner of any page and then click the shipping button. Lots of good info there.