I always consider the dealer price on a used guitar to be about 20% higher than a private party price. I attribute this to "inventory costs", i.e., the cost of having the item in inventory for how ever long it takes to sell.
I bought my 1990 Blackstar a number of years ago from a dealer in Florida, through GuitarBase, because it was actually priced at typical market value (at that time) and not 20% higher.
It is better, of course, to know useless things than to know nothing. - Seneca