Now, before everyone gets mad that "lol bro r* wont invent that, its too resource consuming lol noob", I don't really even care about that. What matters is that if you'd like the concept of that.
Secondly, my ideas are not set in stone. If you disagree with my idea, that's fine. But I'd like you to improve it somewhat, if my suggestion is not good enough.
Alright! Now we get to the explaining.
It would be a place held within some sort of a storage (Simeon's garage at the docks for example? Or something more easily accessible?). You could hold an auction of anything you owned, as long as you have bought it or at least modified it. This would allow you to "sell" things that you have at least resprayed or even only added a simple plate. If it was a stolen vehicle, cops would not go after the guy who bought it... I don't know if adding e.g. a different colored plate makes cops not chase the driver.
It would not be limited to cars. If you had bought e.g. a Titan or a Buzzard, you could sell them at the auction too. If you have bought an Apartment, you could sell it there too. If you had clothes or bling-bling you don't want, you could sell them too.
There would be a list of things that have been given there. You could search for the thing you look for, or look at the list itself and view the specific things under category (e.g. Vehicles -> Cars/Aerial vehicles/Boats, Properties -> Garages/Apartments etc.)
The seller would be able to put the thing he/she is selling, and set a time to sell it in real life days, maximum being a week. The person selling could also set the starting price, for billions if he/she wanted to. It would be unlikely that anyone would buy it then though, in that case. They could also set it at $1 for an Adder, which pretty surely would raise competition, and raise the price to new heights.
The buyer would be able to set a price he/she is willing to offer for the thing there is. There could be a notification that someone has offered more. You could offer any price imaginable, if you have the money for it.
The point of this system would be to allow people to get some of the rarer things that spawn (e.g. the bike Nemesis, Buffalo, Blazer), so that they don't have to waste their life searching for those. It could allow the selling of things that you have already modified, for a price you select.
What do you think? Be sure to tell your opinion & improve it if you feel like there is something to improve.