The best cars to sell are the NPC modified vehicles (if you're on PS4). Easiest to locate are the blue or yellow Sentinel XS's, location is posted
here
The blue or yellow modded Sandking XL's currently spawn across the street from
Los Santos Customs at the car lot, appear to be late morning to early afternoon. Spawn times appear to be sporadic and best vehicle to drive is an off road vehicle, and I had one spawn driving a modded Sentinal XS. The blue Sandking (with bedcap) sell for just over 18 grand, and the yellow (with rollbar) sell for 17 grand (I think).