Player wants to play a Stock broker / Trader. How tomplement a lightweight trading system

You're not having fun.

Since the entire point of role-playing is to have fun, you need to do something about it. In particular, it sounds like the character he's playing (called a GMPC) is seriously distracting him from his GM task of running a fun story for the other players at the table.

You have several options, depending on whether you're willing to accept him as a GM as long as he changes, or if you can accept him as a part of the group as long as he isn't GMing:

1. Talk to the GM about his behavior. Try to be non-confrontational, and don't have this conversation in front of the group; that will just make him defensive and won't accomplish anything. Try persuading him to not bring a character along with the group; he's GMing, not playing.
2. Sit out the current game, until the GM is done. There's no need for you to continue playing a game you dislike, so take a break until he's done running the game. Then join the next game, that someone else in the group will run.
3. Take over GMing duties, with the support of the rest of the group. Since they already asked you to be the GM, they'll almost certainly be willing to let you run the game. Talk to the other players about this individually before you try this; if you have enough support, you'll be able to usurp his position as GM, and start an entirely different game. Perhaps the current GM will be a better player than they are a GM. And look on the bright side: you now have some fantastic examples of how not to GM, which will make you much better at this new role! You can also combine this with #2 above: sit out the game until it's over, then be the next GM. (Warning: given what you've described of his personality, the current GM is unlikely to take this well.)
4. Start a separate gaming group, inviting whoever is willing to play without the problem GM. If some people are unwilling to play with the new group because they don't want to exclude the current GM, then you'll need to exclude them as well. Recruit a few other players from elsewhere if you need to.
5. Find a new gaming group. If all else fails, just don't play with this group at all. If they're willing to play with someone they dislike, and you can't persuade them to kick out a problem GM, that's their problem. It doesn't have to be your problem.

Note that none of these options include playing a game you dislike with a GM who you hate. He isn't going to improve on his own, unless you talk him into changing or encourage the group to leave him behind.

On a slight tangent, see the Geek Social Fallacies. It sounds like some of the members of the gaming group are suffering from GSF #1: "Don't exclude anyone" and GSF #5: "Failing to invite someone is a deliberate snub."

There are two ways to deal with this that I have experience with. Which is better depends on your situation, but both are workable.

Ask for a list of houserules.

For me personally, I have a really big problem playing in a system where I can't know all of the rules up front. If I'm playing in a game where there are significant house rules, then I'd ask that they be written down before play, so I know what I'm getting into. I'd prefer that such a list was handed out before character creation, so I can know if I'm going to need to change around my character based on the houserules.

I can understand why asking for a list of rules might seem pushy, but it's important to know the rules of a game when you want to play it. If you don't want to ask for a written list, maybe just ask for an explanation of the houserules, rather than a written list.

Let it ride.

Sometimes, the kind of house rule that the GM is using isn't the kind of thing that you really need to worry about. For example, you say that your GM is going to use the 4th ed skill challenge rules. Practically speaking, a player doesn't have to know how those work. All the player needs to know is that they are making a bunch of skill checks to achieve their goal, and that their normal combat powers aren't going to be of much help. That kind of conflict resolution rule can be handled entirely behind the screen, and doesn't really affect how a player will play the game, or build their character. If you can be confident that the GM isn't going to suddenly change how your character works in the middle of the game, then it might not end up being a big deal that you don't know all of the rules that they're using.

The downside of this approach is that you can't really get attached to the mechanics of your character. This approach assumes that you can play your character narratively without being terribly worried if one of your abilities stops working the way you thought it would. If the mechanics of your character are important to you as a player, then this approach won't work well for you.