Note: This isn't a new glitch, I'm not the originator. This isn't a standalone money glitch in itself, but most (all?) moneymaking glitches rely upon it. Use it in combination with the money guides in the pinned megathread.
I find that people can dupe their bike or car easily these days, and there are 3 or 4 methods on this sub to achieve this. Unfortunately they all rely on the Karin Rebel glitch to make the dupes sellable, and this part is much more complicated. Seeing as it will likely be gone in a week or two when 1.13 is released, I thought I'd post this part in much more detail to help anyone still struggling with it.
Ok so here is the basic version and the detailed version, step by step.
Be on your duped vehicle
Buy a Rebel (every time)
Drive to LSC, scrape vehicle on the way
Put front wheel inside LSC door
Phone mechanic, when he answers drive inside
Stop at LSC logo on floor
Select Rebel. You now have 20 seconds to get to the sale price
Push right on d-pad, select sell but don't confirm, within 20 seconds
When orange save circle in bottom right appears then disappears, confirm the sale
Run to your Rebel, drive it back into your garage but it will disappear.
Be on your duped bike or car. (The dupe must have been purchased any time after the 1.12 update, any earlier the sale price will be bugged.) To make sure it is the dupe, get off it. If a personal vehicle icon appears where you are on the minimap, it is not a dupe. You must be in the dupe rather than the original vehicle, because otherwise in step 5 you will not be able to request another vehicle from your mechanic.
Pull up your phone and enter the Southern San Andreas Super Autos site. Buy a Karin Rebel - this is used because it is the cheapest sellable vehicle. You have to buy a new Karin Rebel before every sale, because this glitch is tricking LSC into thinking your duped car or bike is your Rebel. This Rebel will be sold and disappear from your garage every time.
Drive to Los Santos Customs (LSC), and bump/scrape your duped vehicle a couple of times, enough that it will need repaired. I usually just slowly scrape it against the wall as I turn into the LSC parking lot.
When you get near LSC, approach the LSC door slowly and it will open. Come to a stop just as your front wheel is inside the door, but not so far that the animation of driving inside LSC is triggered.
Phone your mechanic. When the mechanic answers ('You callin for some wheels?'), the LSC door will begin to close. But because you have your front wheel inside, it cannot close fully. Now drive inside LSC. Because the mechanics list of cars are displayed, you will not cut to the LSC animation ('Whats wrong with her now?').
Inside LSC, there is a large LSC logo on the floor, next to the LSC guy working at his bench. Drive over to it and come to a stop on it. [edit - it seems that stopping anywhere in the garage is fine]
In the mechanics menu, highlight the Rebel you have just bought and select it. This triggers a 20 second window before your Rebel is 'delivered' (the 'time up' signal being an orange save circle in the bottom right hand corner).
It is crucial now to get the sale price to be displayed before these 20 seconds are up. First, immediately push right on the d-pad to repair/respray etc. Because you scraped your vehicle on the way to LSC, the LSC menu should give only two options: Repair vehicle or Sell. Again immediately, select Sell to display the sale price. And that's it. If you have succesfully selected these 2 options within the 20 second window, you will see the higher price of the vehicle you are in. If you took longer than 20 seconds, you will see a low sale price of around $1800 (eg the sale price of your Rebel).
You are now asked if you are sure you want to sell. Do nothing yet, because you are still within the 20 second window. Currently the game thinks you are inside your ORIGINAL, NON-DUPED high priced vehicle. (so if you sell now, it will disappear from your garage forever). You must now wait on the orange save circle to appear and then disappear in the bottom right corner of the screen. As I said before, this indicates that your Rebel has been 'delivered', and the game now considers you inside your Rebel. Now you can confirm the sale. You will be selling your dirt cheap Rebel but be getting the cash for your high priced vehicle.
You will be left standing outside in the LSC parking lot. There will be a PV icon on the minimap and your Karin Rebel will be there. Treat this as an error as you have just sold your Rebel. However, I found it to be ok to get inside it and drive it back to your garage (you will avoid cop problems by doing so), but be aware that once you drive into your garage, this pseudo-Rebel will immediately disappear. [Edit - it may be safer to just ignore the Rebel completely and just steal a car or call a cab home, then walk in the garage.]
Repeat from step 1 every time you dupe.