BETTSY:

For the Ebay user who sold your card, keep track of his feedback. At some point, hopefully, the buyer will leave feedback for his purchase. Once that's done, contact the buyer and ask what the serial # is on that card. If it matches, then you may have solid grounds to get yourself the money you are owed.

For Panini cards, I now make it a duty to scan front AND back when I sell so that I also get the serial # in case I get screwed.

took the words right out of my mouth, well off my keyboard i guess

this is the perfect place to start