You're going to have to be extremely patient. Mine didn't show up again until I was 93% completed with the game and had searched for hours on end for days. The best way to refresh the used car lot is to go into the license test and just load up the first 5 tests and then go back to the used lot and see. Rinse and repeat. It sucks and it a really crappy job from PD but it's the only way. Maybe you'll luck out and get it early though.