I wouldn't take anything to that trans shop and I'd be sure to warn everyone you know of them as well. They wouldn't be able to tell if there was anything in the pan unless they removed the pan and then they would have found the filter popped off and "should" have been able to pop it back on and be good to go. They clearly aren't reputable and only care about $ and not customer service! Sorry to hear you sold it.

I'd even go so far as to call the tranny shop and let them know what you found out from the new owner. Call them out and let them know you know, and then tell them you plan to tell everyone about it as Rex suggested. Ask for the owner or manager and then also file a complaint with the BBB. I'd do everything I could to make trouble for them personally.

That is ridiculous and infuriating.