you don't even have to be rich. it's not much more onerous than a generic small business. i know more than one 2-car-inventory dealerships.

but it's only worth the effort if you are rich enough to buy cars that incur substantial tax and reg fees. you still need to have insurance and the dealership kind of insurance costs a lot. you probably aren't going to get umbrella coverage to include it either, so you'll have more limited liability.