Create a black market where people can sell and buy guns/drugs/c4

[Domingo]

I think having a black market would be a great thing to have. Black markets would allow you to sell guns which you can get by robbing houses, buying/selling drugs, buying weapons at a discount, and buying C4 for city bank robberies (check the suggestion here).

Black markets should be somewhere where players don't usually go so we can boost the popularity for those places.

Great spots would be:
LS -> East Los Santos or Jefferson Motel
SF -> Garcia
LV -> somewhere in the desert

If a cop sees you buying some things at the black market, you will receive 1-2 stars for illegal trades.

[Burger.]

I think this is a great suggestion, I myself would add some ideas to it once I have time.

buying weapons at a discount.

The only flaw I see with this suggestion is that you'll be able to buy weapons at a discount.
Yes, it would be beneficial and efficient for us criminals due to the ammunition store prices. But, it'll get to a point where players would neglect the ammunition store, which is not a good thing.
I suggest there should be some sort of deterrent.
Limit how much ammunition you can buy.
According to your level - you can only buy certain weapons at the market.
If needed - You can only buy weapons at the black market at a certain time In the game similar to the bank opening hours.