What will be the fair offer?
I think "fair" is the incorrect way to look at this transaction because "fairness" implies some sort of rule book we should all be following. Sure we have our own rulebook as to what we would judge to be reasonable, problem that everyone has their own "unique" rulebooks with different priorities, morals, value, and beliefs than ours.
I'm guessing the OP is looking for some sort of number like "average" so he can feel good/secure in having some point of reference, but it doesn't exist as a good reference unlike higher quantities sold per period to make the average more meaningful.
Better to focus on the quality/condition of the speaker, and the seller's motivation. Fantastic deals do happen, but are extremely rare.