If you're looking for a way to earn some quick cash in GTA Online, here's a video that may help. Youtube user Street Reaper has uploaded instructions on where to find a modded Vapid Sandking XL, which can be sold for approximately $17,000 - $19,000, depending on which one spawns. This vehicle can be found in other GTA Online locations, such as Sandy Shores, but this is among the most reliable. This is currently confirmed to work only in the PS4, Xbox One and PC versions of GTA 5.
As the video explains, you should try to find it between 10 AM and 5 PM (1000 hours - 1700 hours if you go by time on the phone), and just keep trying if the military vehicle appears in its place. Once you spawn one and keep it, you can use it to keep spawning more. Since you're allowed to sell a vehicle once each GTA Online day, or about once every 48 minutes of real time, this is a good way to build up a good supply of money.
What are you favorite GTA Online vehicle spawns?