You guys are eBay noobs, I would have trusted that guy. It's true there are plenty of scammers about, but people who put every good deal down to something illegal are just gonna end up missing on bargains. Plus... it's covered by eBay Buyer Protection, so even if he did get scammed, he'd get all of his money back.
My only question is, if someone reports that code not working to steam and they match it to your account, couldn't your steam account get banned?...