sold a transmission now having buyer problems

This is exactly why I don't sell things.
yep.....

I run a shop and hate putting in used transmissions because 3/4 of them have problems. And I never, never, NEVER put in a used transmission without a new front pump seal, rear seal and pan filter / gasket. I don't want any issues coming back to haunt.

I think the shop putting in your transmission has a share in the blame, so if it is a leak I put it on their shoulders, or the buyer for not checking seals on a used part. If it doesn't shift, you need to give them their money back.
Buddy owns a trans shop and has a lot of people that ask him to put a wrecking yard trans in, often times so they can just sell the vehicle. His policy is the same as yours in regards to parts. But then it is only covered against leaks. Only thing that is warranted is that it will not leak and it will not fall out of the vehicle. Beyond that, if the trans is bad well you are sol. And he usually will not install a used trans unless he knows you.....