I realize the OP is asking for an actual percentage of retail, but as Logical pointed out, there are too many factors to be able to give a "rule of thumb" answer. Because of variables associated with each individual buyer and seller, fair market value changes with every potential sale.
When buying or selling a used item I generally apply the principle that states, "The fair market value of any item is the intersection between what a knowledgeable, willing, and unpressured buyer would pay and a knowledgeable, willing, and unpressured seller would accept." In practice this usually works out to a range of value. For example, let's say I have a cue I am asking $500 for, but privately I am willing to let it go for $400. A prospective buyer wants the cue and offers me $375, but privately he is willing to pay up to $450. The fair market value of the cue in this potential sale (and only in this one) is somewhere between $400 and $450.
If you can't come to a decision with your buyer on this particular cue, you could try something I have used with good results a few times in the past. You write down the least amount for which you are willing to part with the cue. Your buyer writes down the most he is willing to pay. Before you show each other your numbers you agree that if your number is lower than his you will split the difference equally and complete the sale. If your number is higher than his, and neither of you is willing to change your number, just walk away from the deal.