Okay in the end! There was a kerfuffle where the key had already been used, but they contacted the site's customer service and, after I sent a few relevant screenshots of my library and purchase history, we got it sorted out quickly. They also offered to buy a new key if we didn't hear back within 48 hours, which was very sweet.
+rep super fast and reliable trader! :)
EDIT: ...and after that trade, there were immediately other two! I've never traded so much in the middle of the night. :D