i have to disagree with the , "you can't expect to get full price for them", its not like because someone else owned them they are half as good or worn out, that would be like saying a dog breeder cant expect full price for their dogs since they owned them before they sell it, same thing basically, if the fish is in good health then you can try to sell it for what its worth, the full price of a fish is usually doubled when on sale in a fish store and a lot of those stores take lousy care of their fish and people still pay for them.
i would rather pay a few dollars mroe or even what i would pay at a fish store to get a fish from someone if its healthy and been in their tank for a while and all that, more likely to survive then one from a store like petco or soemthing, dont go low just because he doesnt want to pay, let him go buy them from a store, hell prolyl end up paying more then 64 dollars for the fish and they might not be as healthy... just a thought