IMO the omission of 'backup' is an honest mistake, and if this is the reason for his complaint then 18 months is stretching it a bit.

This scenario would apply to any chip which came down in value over the period, for whatever reason. Buyers should do their homework and know if there are gazillions of them in existence before they bid

To summarise - "sometimes $hit happens"
IMO you were being very generous offering him $200