PucaTrade users create a "Have List" and "Want List". Each card has a PucaPoint value and when you send a card you have to somebody who wants it you earn that many points. Others in turn send you cards on your want list in exchange for your points. The site is free to use and is currently raising money to break out of beta mode and add some new features.
For more details check out PucaTrade's website, their press release, and their IndieGoGo page (which includes a video).