I don't know what dealers do. Dealers have a brick & motars to pay for so most tend to low ball so that they can make a profit selling. It's business not a friendship. I use coinflation when dealing with friends. I think its fair to pay them full spot because I'm not going to flip their coins, I'm going to hold onto them, maybe add some to my collection so we both come away happy and feeling good about the transaction and the friendship. Dealers don't care about maintaining your friendship.
A long time curator of the National Gallery had a sign above his desk "We Collect Collectors". It is my experience that people do buisiness with those they like.
Clembo has it pegged. 90% or more of the dealers would do the same. The rest will offer to pay even less.