in such the short time between the sale and the complaint, sounds like the buyer may have made a purchase they couldnt really afford and need the money back. maybe when they got home there were a heap of bills in the mail, or he realized that xmas is closer than he thought, but this is one reason if i was a seller i wouldnt be doing pick ups, when a sale goes bad, who knows what the buyer is capable of.

hope your buyer is ok, and you get it sorted, but from what i have read i wouldnt be accepting a refund, unless they were able to provide written documentation from an independant music / guitar shop / guitar tuner stating there is an actual fault with the guitar