If you didn't mis-sell it, its his problem. The colour was wrong in the photo? Get real. The decals are a bit too lifted? Grow up, its a bike not the mona lisa.
So you got negative feedback, someone doesnt love you, so what – it is gonna change anything? is it gonna stop people buying from you/you selling? I have one negative feedback in 400 positive, and that was from an unreasonable moron. Who cares, it makes no difference to anyone.
Just ignore him. In a weeks time you both wont care.
This is a lesson to all DO NOT ACCEPT PAYPAL payments for goods. In this situation paypal would just refund him and you would be screwed, plus fees!