A quick tip if you want the User Car Salesman achievement, but don't have much cash. Buy a Chili, then go to your Garage and sell it straight away, rinse and repeat. You lose money on each car you sell, but with the Chili you only lose about 2000 credits each time. The amount of cars sold is total, you don't have to buy 10 different cars or sell them all at once or something, so just buy and sell a Chili 10 times.
As for Car Rental Service, just pick up the 6 cheapest Derby class cars, (then you can always sell them all to add to your sold total). You shouldn't lose more than 15000 credits on the deal.