Take it to a small claims court.
He wouldn’t stand a hope in hell.
He bought it, then drifted out the BB bearings (and stripped the paint). The seller could quite reasonably claim that the frame was fine when it left his possession (as evidenced by the buyer picking up in person, having a chance inspect and reject) and has been damaged by some ham fisted spannering. This could even be true! The buyer did not realise there was a problem until after the BB was out – could you have done the damage OP?
The seller could reasonably claim his original offer was in hast until he thought through the possibility of buyer inflicted damage.