My take here is that the tranny was taken to a tranny shop and that the seller was told all is fine. Why it would not have been all buttoned up is beyond me. If the tranny is good, why leave it open to be looked at?
If the buyer was aware the tranny was open at purchase and saw there was issues upon receiving the package, then it should have ben dealt with. If the buyer sealed the tranny, installed it and used it, then it is a used part. Who is to say the buyer did not create the problem sealing the tranny up??
Hard to take sides when there is not 100% detail from both ends.