When I dealt in antiques the understanding was that you let the seller set the price. If he is willing to take $100 for something and you are willing to buy it for that price then everyone if happy. You are under no obligation to tell him it is worth $500. One valuable lesson I learned from the guy who set me up in my current business is that the first person who speaks in any business deal loses. Think about it and you will see that it is true.