UPDATE: GLITCHING LOS SANTOS CUSTOMS
You can still glitch the Los Santos Customs post 1.04.
Get an Ubermacht Oracle and max it out with all the mods you can. (now it sells for about 110k maxed out)
Go to Los Santos Customs but don't drive in.
Change your options to spawn at LAST LOCATION instead of appartment/garage.
Exit GTAV Online.
Go back to GTAV Online.
Drive the Oracle into Los Santos Customs.
Sell the car but immediately go into your pause menu and ONLINE-> SWAP CHARACTER
Pick your same character (you only need one char for this anyway).
You will get the money and respawn inside the car somewhere near Los Santos Customs.
Rinse/repeat. 1mil every 10min if you have racing maxed.
Without racing maxed out, you get 40k or so per swap.