I had just a small thought (I know small minds, small thoughts) anyway.

I was thinking - what if your DFPs could buy drugs as well as sell them? I mean in real life you would not just be able to produce and sell drugs you would middleman too right? Right now it's set up so you can buy from a DFP and sell to the drug dealer. What if you could set it so you are willing to buy your DFP drug type at a certain price. Say the price of cocaine from the drug dealer is like $5500, a local DFP could be willing to buy the coke for 6k. You could have a cap on it too. You could buy a new 'warehouse' upgrade, and once the warehouse is full that DFP cant buy anymore product. 

On the other end of things. You've got a DFP with a warehouse that holds maybe 40 units in it. You're offline or in a different city and your home city's drug dealer starts selling cocaine for really really cheap. So the people in your city buy up 40 units from the local drug dealer and sell it to you for a set price. So you get cheap coke, and they make a few bucks really quickly. Obviously you would have to leave money in your DFP so that you can purchase drugs. Or maybe it could be auto-deducted from your federal/local account (would make more RP sense if you could just do it from your city bank account) 

So anyway, just a thought I'm throwing out there.