Ricky,

The issue is what obligation do you have to correct an error 18 months after the fact. The answer in my opinion would be none, unless you intentionally attempted to defraud the individual.

If you didn't know, or forgot in good faith, there is no intent to defraud. If this were a few weeks, you might have an ethical obligation (regardless of the actual value) especially if the buyer lacks the level of knowledge you have. Unlikely that a buyer would spend $600 without some knowledge.

I don't think you owe the buyer anything at this point. UNfortunately, don't be surprised if this buyer takes the low road. If he is devious enough to try and wiggle the $600 out of you now, he may be devious enough to try and shame you into acquiescing.