Information brokers show up in almost every story that involves some rogue-like character. The person, well placed in a given city, whose job it is to specifically know everything there is that goes on. This could be someone as far reaching as the Shadow Broker in the Mass Effect Universe, someone who literally has his hands everywhere; or as simple as the Chief Thief in a thieves guild who makes a point to keep his ears open. The new Game of Thrones show has made a point recently in showing off a number of information brokers, so let's take a look at what you need to make one.
Not For Beginners
The first thing you need to know for an Information Broker, is that they're not a character that just started out. Setting everything up they need, harvesting the contacts and resources needed, takes time. Often, the character had the life of some sort of adventurer or some much smaller piece in "the game" before becoming the broker.
This means that you shouldn't be playing a broker at level 1, but don't let that be a downer for you. These early years can be some of the most important in your broker's formation. You'll need the time to make your contacts, get things in place to inform you, and build your entire network.
The Network Is Important
This is something that can be a bit of a downer, but as formidable as an Information Broker can be they are nothing by comparison without their network. As much as a person may know on their own, you can only gather so much of it in one go.
Now, how you make that network is up to you. Some characters, like Batman, just monitor everything going on around them. Bugs, hidden cameras, and some old fashioned detective work are how they stay informed. Others, use networks of people. Spies and agents that infiltrate every layer of the local infrastructure. It should come as no surprise that in fantasy settings, or other low-tech settings, children can make great resources for this. Very few people pay attention to children, and they can get forgiveness for going places they shouldn't. All the while still having sharp eyes and ears to learn whatever they can and report back home.
As An NPC
As an NPC, an information broker can be a big help to a game and a valuable resource to the players. This is, generally, the easiest way to handle one of these characters. The PCs go to the NPC, pay a fee and get their information. As to whether or not they trust that information, or what the broker's ulterior motives may be, well that is between you and them.
As A PC
As a PC, things get considerably harder. There is a lot of information in one PCs hands, and in the kind of games that a broker shows up in this equates to a lot of power. This isn't necessarily to be feared, but the problem lies in how you handle the PC getting the information. The whole point with the broker is knowing everything, which means either telling the player a head of time (in which case you need to make up a bunch of useless information to go with the real stuff) or giving the player the information when they ask/it becomes relevant.
Honestly, the best way I've seen this handled so far is how Mutants and Masterminds handles it with the "Well Informed" feat. When first meeting someone (or if a great deal of time has passed) the player gets to make a Gather Information roll. Then, depending on the roll, the GM gives relevant information. This saves the GM from having to put a bunch of information out that may never be needed/useful, and also lets the PC have the fun of being a broker.
Power in Secrets vs. Need For Neutrality
One of the things that should be pointed out is that information brokers often have to be neutral. There is power in secrets, and that power can keep the broker safe. However, if a Broker is known to take sides, then there is a good chance that someone with a lot of guns isn't going to care about those secrets and just take the broker out. There are ways around this (anonymity, secrecy, etc), but the best solution - and what many information brokers take - is a policy of neutrality. They observe, they give information, but they do not take sides. For this reason, they can simultaneously be the most dangerous and most helpful of allies.