Go on to the auction page and search for a car (with unlocked graphics/design) getting a lot of bids.
Buy it for the buyout price and save the design in the design catalog.
Find some cheap models of the same car you bought and put the design on all of them.
Then sell them (all at different times) in the auctions and remember not to lock the design as many people only buy cars for their design.
You should make alot of money by doing this.

This only works with cars with designs which are not locked!