Specifically High End cars. You'll make the easiest money you'll ever make! And the pay is SO amazing during the very rare events that pay double.
Add to that if you fill the warehouse with standard & medium cars but don't sell them at about 80% full every car you get from the source mission will now be high end a possible $80,000 every 20 minutes & plenty of time to get another & fit in some CEO work while you wait.
and make sure not to have any car twice in your garage. its the 10-10-12 rule.
you have to own 10 standard class vehicles, 10 middle class vehicles and at least 12 upperclass vehicles.
Not at least 12, but a maximum of 12. When you source a car and the warehouse is 80% filled cancel out and go sell one first or you will have a chance of getting another standard or medium range car. Basically keep eying the fill percentage and when you are sourcing and it says 78% You will know you have to sell after stealing this one.
I also keep two collections for when I get friends to sell. I only sell the collections which consist of only top range cars. When sourcing a car that is part of those collections I keep it and sell something else instead. When sourcing one of those cars but with the wrong plate it is a higher priority to sell in order to increase chance of getting the correct one.
Top range only collections are:
Rooselvelt Valor: OLDTIMER
X80 Proto: MAKEBANK
I mostly use a cargobob. The X80-Proto and the FMJ are two of the easiest to grab with it, which is important for speed and getting them out of more difficult corners, unless they are in a garage or under a roof. So when I am trying to get the MAKEBANK X80 I will be stealing and selling only those (since I am usually at 78% fill rate) and then switch to the FMJ when I get the right X80. After selling a collection with friends I will work up to the 78% again and completing the collections again before returning to the FMJ sales.