Edit 2. Grab Quick Cash

As a change of pace, you can also knock over a convenience store or steal a car to pick up extra bucks.

Car Theft has a pretty great return, although you can only perform thefts infrequently. Robberies can be fun, but they're dangerous and not particularly cash efficient.

Edit Selling Cars

Los Santos Customs Mod Shops will purchase non-tracked cars for handy prices. Driving a jacked car to a nearby Mod Shop will garner a little extra spending money. Note that cops online are always on the lookout for stolen cars. The rewards can be substantial, although you can only sell one car per in-game day (48 minutes). SUV's sell for good prices, ranging between 3,5K and 7K (Baller is worth 9k). Lampadeti Felon's sell for $9,000+. For more Information view Best Selling Cars. The math appears to be that the modshop sell price is 10% of any vehicles actual purchase price (which is why no car can be sold for more than $9,500, upgrades notwithstanding).

Repair prices match to the discounted 'sell' prices (ie. Damaged car will sell for $2,200, repair costs $300. Repaired car sells for $2,500). 

It is also worth considering using your second character slot as a 'car mule'. That is because all banked money is shared between your characters. but any one of your characters can only own one property (for now). Therefore, invest in a cheap 6-car garage for your second character (preferably near a mod shop, for labours sake). At the start of any gaming session, use your mule to go off hunting for any $6-9k cars. filling your garage. Find a seventh car, sell it. Now switch to your main character, play as normal, then roughly every 50 minutes, you can switch back to the mule and casually sell the held cars. Of course the additional benefit (particularly at this point in time ) is that switching between your characters does a 'manual' save to the Rockstar vapour servers.

The Richman area is probably one of the best places to prowl for the high-end (but sellable), cars.

Simeon also has a varying wish list of high-priced rides. He will text you a list of cars he desires. If you happen upon one of these, take it to a mod shop, respray it, then drive it down to the docks. Bring them to him, and you'll garner a decent reward (approx. x2 what a mod shop would offer depending on the condition of the vehicle ), even if you are left stranded in the docks.

Edit Robberies

GTA Online Robberies are similar to GTA 5 Robberies. You step into one of the many potential stores (which include gas stations and different convenience stores), point a gun at the clerk, grab the money. and flee before the cops arrive. Online, a couple of new twists are added to the process.

First, yelling at the Cashier causes him to unload his money more quickly. Note that your yelling may be overheard by others on your chat channel. Second, Robberies may be conducted cooperatively with other players, with the bagman responsible for divvying up the cash at the Heist 's conclusion. To divide the earnings, you hold select to bring up the Quick Interaction menu. From here you go to Inventory->Cash. After this you merely click the split option, and divide the money amongst the participants. Note that cuts are in percentage and not raw amounts for convenience. 

UNSTOPPABLE GETAWAY STRATEGY:

This method requires 2-players, but if done correctly, you will be able to hit every store in San Andreas multiple times while barely taking any damage.

First, get a Three-Star Wanted Level. Next grab a police van. Have someone driving and someone in the back. BE SURE THAT THE DOORS ARE CLOSED! The police van is covered in armor and has bulletproof windows. If you back the van up to the doors of the liquor store leaving just enough space for your partner to open the doors and get out, you can hit every store in San Andreas. making an easy 15 grand at least. The hard part is losing your wanted level afterwards as the van is incredibly slow.