It depends on your Player. One thing that keeps coming back to me in reading this thread is the old Woldside sourcebook for Fixers in Cyberpunk...
I think there are likely to be 2 black markets on a frontier station, especially with a Starfleet officer from the Ferengi.
As Nog showed us, there is a strong system of 'trading favours' within Starfleet, where you have to navigate the deals and coordinate the product until you get what you want... For which the 'Great Material River' is a childhood lesson for the Ferengi that puts them several steps ahead of all those hoo-mons...
The other black market is going to be the dodgy one, with the cash deals in dodgy locations, where you have to watch out for undercover law officers and second rate product... Where the item aquired may be stolen and may even draw more danger by its meer presence. And where a deal could easily turn sour and leave you with nothing except violence...
Now like the 'favour-trading' a Ferengi is again, naturally several steps ahead on this market too, most especially as many of the front men are likely to be Ferengi too, and the Rules of aquisition clearly support a deal between Ferengi as honourable... 
"I may be a Starfleet officer. But I am also a Ferengi."
The levels of black market and people your dealing with will vary depending on the product;
Small deals
Likely to be expensive jewellry, electronic goods and luxury items.
These are likely to be one offs in a public place, and not always very private. Its a good way of making contact,and generally ignored as the small individual deals are almost inconsequential to the local law enforcement, unless there is a crime crackdown, but on the whole the most that will be done is a confiscation of goods. Expect to pay between 30-60% of the stadard price (depending on product pedigree and demand). Also expect that much of the produce is fake, stolen or smuggled.
Bulk Deals
Similar items to small deals.
Buying in bulk. A large package full of a particular item, definatly more than 1 of the items in question. Usually purchased by people expecting to sell the items on themselves. There is an increased risk of Law Intervention and arrest and seizure. But the profits are slightly better as the product is generally 'cleaner', ie; not stolen (as such) but more misappropriated or damaged stock. This is the source of the Traditional 'back of the lorry'. Expect to pay 40-50% on the deal, with the possibility of price mark-up for re-sale.
Stuff of life
This end has the unscrupulous business men making a profit on misery, such as food sales in a famine, or pharmaceuticals along the front line. There are very few 'Robin Hoods' here, these people are selling a product that will save lives, your life, if you can pay...
This is the dangerous end of the market (for the salesman), if word gets around things can turn ugly, and they can even get dead in a hurry. But if you get in, make the sale and get out before the Law or Military get you, a great profit can be made. Expect to pay 200%+ for standard everyday products that are in short supply. Often this type of Marketeer will have his own security to ensure that the crowd doesn't get too rowdy and poses no threat to the profits.
Hardware
Military and Weapons technology.
Full on military hardware, clean, untraceable and perfect for your needs. This marketter is a gun runner, finding and supplying the illegal weapons sales, supplying pirates with the weapons they need to hijack the spacelanes, anything from a Phaser Rifle to a Spacecraft. Expect to pay 500%+ Price subject to change at a moments notice, and expect to be surrounded by dangerous goons with better weapons than they are willing to sell on.
You can thank both 'Wildside' and my uncle Carlo for this info, based on Londons black market.
DanG/Darth Gurden
The Voice of Reason and Sith Lord
“Putting the FUNK! back into Dysfunctional!”
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"Ad astrae per scientia" (To the stars through knowledge)