There are many methods of doing this, but the one i've discovered to be the most efficient is this one which takes only 10 seconds. THIS WORKS AFTER 1.03 PATCH
1. Go buy the LCC Hexor online for 15k. (MUST BE THE LCC Hexor)
2. Go to a LSC and upgrade it to the max (Much more efficient if it were to be 40k+)
3. Switch to Single player mode (So your car can be in your garage)
4. Switch back on to INVITE ONLY SESSION
5. Get the LCC Hexor to a Los Santos Customs
6. Once in there hit start and L1(LB) and press X(A) twice.
***DO NOT REPAIR, BUY OR SELL ANYTHING BEFORE DOING STEP 6
***DO NOT SPAM X(A) WHEN ON STORE SCREEN
7. Once the store loads hit O(B) and you should be taken back to the garage on your bike with the menu up.
8. Leave the garage and then re-enter.
9. Now proceed to sell the bike and quickly hit start and L1(LB) once and Press X(A) twice followed by O(B) once after you have sold the bike.
10. Keep repeating this and I gurantee you will make atleast 300k in 10 mins.
***THIS DOES NOT WORK FOR CARS OR ANY OTHER BIKES
I know some of you are gonna be giving me crap for this, but i bought the game so i'll be playing it how i want to.