Did you check the OAT EULA? My experience of places like ebay is that they have a big load of spin ready to fire off at the first wind of trouble which is probably what you got in that email. It is easier for them to close the auction rather than argue back and forward. Can you imagine them ever actually reviewing a copy of the EULA themselves - yeah right! Probably best to try and sell it privately by going back to a flight school if you are confident that the rules aren't being bent.