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 10 Pro Gigabyte 970A DS3P FX AMD FX6300 Vishera 3.5 Ghz ASUS STRIX GeForce GTX 970 Overclocked 4 GB DDR5 16Gb Patriot Viper 3 RAM DDR3 1866Mhz Onikuma Gaming Headset (has annoying blue lights I don't use)