My thoughts are if I bought the tranny I would have opened it up as soon as I got it and looked it over, if it had a problem deal with it asap.
If I sold the tranny in good shape and had someone check it out before hand I would post the shop that did the work and give info for the buyer to talk to them.
I think once a used part is installed and run it is yours. I don't know the truth either way and am not siding with either person just my thoughts on the matter.
Curtis