Any honest seller will research the product before selling. Technically they can do whatever they please, but any decent person will usually stick to their word.

I sold a Mike Melton frame a while back and I offered a lower price to a potential buyer. I got a second buyer who really wanted it at the higher price, I gave him the 50.00 off even though he did not know that was offered to some one else. I deal that way because I want to be dealt with that way.