I've been getting decent money from a car duplication glitch. What you do is do a mission where you and another player are in your customized cars. Switch cars at the start of the mission. Complete the mission in the other players car and then vote to go into freemode. Once in freemode you should spawn in the other players car which you can now sell. Some missions don't allow you to do this however its iffy at best. Trial and error until you find missions that will work