[quote=@Vilageidiotx]
Merchant states tended to be Oligarchies. 

It can definitely happen in a medieval context. Most medieval style states were governed through power by land ownership, but there were some places where local land lords could not compete with the merchants in terms of practicable power. Renaissance Italy is the most famous example of this, though Germany had examples of this phenomena as well. This was especially common in Europe because of the importance of eastern trade to the European market. Theoretically, most Merchant states begin with the nobles being supplanted by the Merchants. It might not be some shady illuminati type thing, but shady illuminati type things are for people who don't understand human behavior and need to simplify it my creating singular hive minds.

If you are going to recreate a medieval version of this, you have to boil these things down to their essentials. Medieval people do not have savings accounts unless you count the coins they sew into their mattresses. High level economics centers around your ability to give out loans and your relationship with long distance trade. Insurance is the guy selling binders on the dock. Instead of a stock market, you have merchants banding together to invest in a voyage in exchange for an equal split of the profit. Banks are guys in the market writing checks that look more like IOU's than anything else, which are being purchased (with interest) by travelers afraid of being robbed and can be exchanged for money at any banker in league with the check-writer (and he has friends in ever important port, so you don't have to worry about that. If he writes too many false checks he know's he'll be stabbed by some sailor anyway, or hanged by the captain of the guard if there are actually laws in place about this sort of thing).

If you want to do a shady company style plot, I would go with the bankers. Look at the Templars, and the rise of the Medici's, to see how this is done in an old school context. Remember that this is a world devoid of Adam Smith's. There are no Free Trade Agreements, and we don't have the communication systems that make large corporations doable. It's unlikely anybody in this culture understands simple economic concepts like "inflation", considering how Rome fought their inflation by hanging coin clippers and minting more coinage because they thought the material in the previous ones must have been inferior to have depreciated in value. I am pretty sure one of the biggest parts of the Medici's rise to power is that they were an early adopter of double entry bookkeeping (to simple modern folk, medieval Quicken.) In this culture, "interest rates" aren't controlled, but are something you haggle with the town Jew over if you need a loan for a set of armor to go-a-crusading with (you'll pay him back with a loaf of sugar from the holy land when you get back because that shit is in vogue now).

I use Europe as the model because I don't really know how this worked beyond Europe and the Middle East. I would assume India worked the same way (minus a town Jew), but I don't recall ever hearing of any banking systems that were not European. I'm just spitting this shit off the top of my head though. You might need to google for the details here. 

EDIT: Black markets weren't much of a thing because there wasn't enough regulation to make them practical. Simply put, you need shit that is both illegal and affordable for there to be a noticeable black market for it. 
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This is very cool and helpful information, so thanks for that. The banking seems like a good practice, especially if that banker has a far and wide influence, and would be good for a merchant after they got their career off the ground. But assuming the merchant is already wealthy: wouldn't it also be viable to go around and place investments in small private businesses? To get those businesses what they need and off the ground in exchange for a share of their profits. Going around and doing that for multiple merchants eventually leads to you earning income from all over the place. You hire a guy with a wagon to pick up the cash and some mercenaries to escort him, they come back, pay them, and now you have a pile of currency in your basement. Eventually, especially for viable business ideas, you can buy them out.

A corporation might not work well, because like you said, there's a lack of effective communication. But you can still own plenty of different shops and whatnot, also act as an investor, and monopolize an area of trade in particular regions.