sold a transmission now having buyer problems

It was a used transmission, no guarantee or paperwork saying this was working or rebuilt. His loss. I try to make everyone happy too and keep the peace too but what if it was a 10,000 used car and he revv'ed it like a jet engine on the way home and popped the motor? would we be even talking about giving him a refund? NO! Anything we buy used wether it be ebay, CL or FABO there is no guarantee. every auto trans I ever bought was gone over or rebuilt before ever touching the vehicle it was intended for. I think that's just common sense.