Once a month roughly, Upland does a round of car sales from their manufacturing plant that players can register for a chance to buy. There are limited quantities so even though they do them monthly they do switch it up a bit with the car makes and colors, etc. just to keep the markets relatively spread out for the time being. Eventually there will be a lot more cars out on the market but it is nice to score a cool car every once and a while, especially now that they do have some utility with the upland racing circuits.
I'm still not sure exactly what the racing looks like or how to do but I am applying for my own car shop in Upland so if I get that I'll have even more uses for trading cars like these.