I live in Khon Kaen and we have a lot of fairly large second hand dealers in the city, it looks like they take the trade ins from the main dealers, but again these don’t have prices on the screen/ windshield, I bought a car from a teacher in the university when I went to view the car I took a small OBD2 code reader with me after the test drive I plugged the reader in and got zero fault codes so I had a pretty good idea if the car was good or not, I ended up buying the car after doing a few more simple checks, just remember Thais are not good at getting basic maintenance done, the tyres on the car I bought were over 7 years old ( tyres have a date of manufacture stamped on the sidewall) good tread but the sidewalls were starting to crack, the second hand car market is a minefield no matter where in the world you are