Depends. Ethically speaking, if you are an "expert", you have an obligation to the novice. For example, little Billy comes into your comic shop with Grampa's Action Comics #1 and wants to sell it for $20, you need to tell Billy its actual value. Some states have laws on the books to this extent.
That said, I think fair-market for the expert would be the wholesale price which is typically 50% of the retail price.
What I hate the most is when some flipper starts to whine about the seller who finds out their bike is worth more than they originally asked and then wants the fair-market price.
That ethical standard might apply to a business transaction, but I really don't think it does apply to casual sales like OP is talking about.