I checked his feedback, and he does not seem to make a habit of this sort of thing. It may well be an actual fault?? you admit yourself you have hardly ever played it, so if there was a tune holding fault, you may not have been aware of it. Maybe just accept the item back, less your listing fees, fvf, and the $20 you floated him

When you pick up an item, that is the time to inspect it for any faults.

When you pay, you have agreed to the items condition, and accept it as it is. The seller here, is not shop.

Mixed tape says the buyer played the guitar for 20 minutes.

(Strum a guitar once, and you can tell if it is out of tune.)  If he managed to play the guitar for 20 minutes, not find fault, then pay......it sounds like buyers remorse. 

Who knows what he has done to the guitar in an attempt to tune it.......a red dot is not going to change your life.