You did nothing wrong. You are not a car dealer. You presented the information and let their "mechanic" look it over.
It was 5 years old with 105K miles! You had no reason to expect the tranny was about to die (right??), nor do you have any reason to believe it needs to be replaced now (the AAMCO Syndrome). Maybe they are driving it and the slipping annoys them... it was doing it on the test drive and their mech said not to worry. YOU drove it that way for over 40K miles.
That said, anyone can sue anyone, at any time, for any thing!
good luck
tom