Here is a 30 second legal analysis (not advice):
1. Civil rights suit - N/A If memory serves only applies when a government official under color of law violates an individual's civil rights. No bearing in a private transaction.
2. Age discrimination claim - most likely N/A as generally applies in an employment context. Also don't see that being relevant to the sale of a firearm or any other good.
3. Obligation to sell firearm - No obligation that I am aware of to sell anyone any good. They can assert breach of oral contract, I suppose and seek specific performance. Specific performance is only available when no adequate monetary remedy. Thus, likely damages are difference between FMV and cost to obtain replacement.
I haven't dedicated any thought to this more than the time it took type out.
