GTA Online: Cars, tuning | Cars and Vehicles GTA Online guide, tips
Last update: 19 August 2020
To be able to use all of the tuning elements, you need to win races in the online mode. By winning a race, you unlock elements for a specific vehicle type (to unlock bike parts, you need to win bike races).
In LSC (Los Santos Customs) garages, you can also sell cars for decent cash (ranging from several to a dozen-or-so thousands- depending on the car value). However, you cannot do this all the time and only occasionally (more or less, very hour)/ For this reason, try to deliver expensive cars only, but not above the super-cars threshold. To learn about this, see below...
Special vehicles (police cars, fire trucks) cannot be sold, nor modified. Also, cars of the other players count as special vehicles. You can often get into them, but you cannot keep them or sell them. However, the groups of special vehicles are super-cars, which you can see on the roads and on parking lots. You cannot sell them, keep them or modify them in the garage, either. What is even more, if you steal such a car, it is possible that you will be hunted by the owner and you get a bounty on your head. To have an easy access to all the cars, the easiest way to do this is buying them. You can buy them over the internet. After that, the car is delivered to your garage and it is supplied with tacking a chip.