Finding a best deal formula for trade and buying

Years ago I tried buying a very nice used 5th wheel off a fellow who knew he had a well kept trailer. He was asking $17000. I offered him $14,000 but there was no way he was going to come down that much as the dealer was going to give him 16000 trade in value. So we parted company and he traded his trailer in. I bought it from the dealer for $13000 which was only $500 under their asking price of $13,500. You never know how much a dealer will move or what they are up to.