That's very unprofessional. It's allowed unless the account details were already given to you. The trade wasn't completed yet.
damn ok i was forced to pay 150 dollars more. since he had other guys bidding higher then a/w. even when i paid.
You weren't forced to do anything, you could have easily backed out and went on about your day. If he told you that your bid won etc and then last minute changes it.. That's not someone I would deal with, proceed with caution.

