When I bought some Bitcoins in a face to face trade months ago, the guy asked if we could go to the bank and get my notes checked out. He had already sent the Bitcoins to my address and I handed him the money and we finished out drinks in the cafe and then he was like do you mind if you come to the bank with me to see if they are real. In the end I suggested him give me the money back and I will get out a fresh set of notes from the ATM machine and he was happy with that.