Wow that is bad. The undercover officer provided multiple mentions of stolen credit cards, and provided enough details (even offering to trade stolen CC for Bitcoins directly) that there is no chance the seller though he was "joking". A single statement might be a joke. Multiple statements with details over the course of two months? He knew.
Yes in undercover stings cops typically make sure they say things multiple times. The excuse of "i thought he was joking" never holds up in court.