Heat is a automatic transmissions worst enemy. A quick check is to let the vehicle warm up for 5 minutes at idle in park. Then an additional 1 minute in drive at a stop. By then the transmission fluid temperature should be about 175*f. Then go drive it for 5-10 miles in stop and go traffic if you hear any type of whining, growling and/or the transmission shifts are out of character of anything that you’ve ever driven, Pass and move on to the next choice. (I’m a 10+ year ASE master mechanic that has seen some very expensive and unexpected transmission failures.) hope this helps and can save you from wasting some $.