Rust in Transmission

My roommate just sold her 1990 Cadillac Deville with 175, 000 miles on it. We posted it on Craig's List "AS IS". The couple that bought the car test drove it and drove it off our property. In fact I watched them as they test drove the car and they hit the accelerator very hard on the test drive out of our apartment complex. I drove the car and it worked fine for me. We even drove it down to Raleigh, NC from Erie, PA and the car never had a problem. It hasn