After reaching rank 10 you will get an access to the missions from Simeon. In these missions you will have to collect cars for him. The list of the vehicles you will have to bring to him will be given to you via a SMS. On a day you can collect only one car. Getting into a wanted car will come with a notification. Now all you need to do is to get the car to the garage in the Los Santos Customs and change its paintwork (choose the cheapest option for about 780$). From there, go to the garage in the docks to finish the mission. You cannot be pursued by the police and if the vehicle is damaged you will have to pay for it, so be careful.
In opposite to the cars you are bringing for sale to the Los Santos Customs, the prices about 1,5x higher, but to the LCS you can bring as many cars more often than once a day. Then again, the cars from the SMS list are the part of the Daily Objectives. The payment oscillates within the range of 2 000 - 12 000$ (excluding the costs of the paintwork)
High priority cars.
Sometimes the special tasks of delivering a car will be given to all of the players. There will only be one car to get, so the fastest wins. Get to the car marked with a green icon on the radar and change its paintwork before getting it to the Simeon. You will be paid twice as much as you would be in the LCS.