it was a pick up item, there is no requirement to accept a return on a pick up item.

who knows what they may have done to the guitar in the meantime>?

email them politely and say, Sorry, no returns as clearly stated in the auction.

sure you may cop a neg. but they could return it and still leave you a neg. no guarantees there.  Stay professional, and do what your gut tells you is right.  NO Returns is there to stop a 'change of mind'.... could you maybe offer to tune it for them??