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

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