I see no reason to pay a guy more for a bike, or anything for that matter, than what the owner has asked. that would just be silly. people make money from their knowledge, there is nothing wrong with that. why do electricians make $45.00 an hour? knowledge. same with wheeler dealers.
a good reason for not doing that is if I had something for sale for $500.00 and someone offered me $2,000.00 I would cancel the sale and go find out what it was really worth.