I bought a lot of vehicles that after a while I sold because they just weren't that great, like the Batmobile which had ungodly acceleration but was made of glass (great homing missiles tho). I have found that the best fast cash solution is selling the cars, once you have stolen them you and 3 buddies can sell 4 at a time and if you don't damage them you'll get $400,000 and after a short time delay you can repeat this. Some vehicles will be unavailable or outrageously priced until you complete a mission with them and then they will be unlocked or discounted. And you'll have to be a CEO for some missions or run or Motorcycle Club for others, like the Weed Farm.
Windows 7 64 SP1 Gigabyte 970A DS3P FX AMD FX6300 Vishera 3.5 Ghz EVGA GeForce GTX 760 SC 4GB 16Gb Kingston Hyper Fury X 1600Mhz Onikuma Gaming Headset (has annoying blue lights I don't use)