Try a couple of things. First make sure you have a Singer needle. Then, take the needle out and turn it 1/4 turn, tighten it up, thread it and try again. Keep doing this until you find where it works. On mine the flat side has to be on the left. Make sure it's the right bobbin.
After that file a greivance with Paypal. If it was sold to be in working order it won't matter if he says "all sales final" or not.