There’s always inherent risk buying a used car, especially a 10 year old plus vehicle. It sucks to be out a car though I feel for you. Not calling you stupid by any means but as I find it doesn’t matter the millage, the dealer or the carfax, a car with no bad symptoms can grenade out of the blue. This is coming from 12 years auto industry experience and I’ve seen this on cars many times, I’ll appraise and it seems/sounds fine and checks out in service. Two days off the lot it drops a trans.