By being a market maker, I mean when I post a limit order, someone could take your order. For example, suppose during this second the bid-ask is 1,1.01 constantly and I posted a limit order to buy at 1. If I am not trading as a market maker, that order would not be filled. If I am, it may be filled if someone else post an market sell order.
The best training I have heard of for this, other than managing to get a job on the desk is ACI Dealing Simulation Course. That would answer all your questions.
The problem you have is that as a market taker, you're right, you can post buy and sell orders simultaneously as if you're a market maker. Who is your client though? If you're posting orders on an exchange and the market moves too quickly, you'll get put into a position when the market is going against you, which you won't be able to get out of without losing money.
This reminds me of General Patton's speech (google it). The point is to make the other son of bitch lose money, not you.
Since the comments answered the equities portion. I'll focus on forex.
First you would need sufficient capital to setup a prime broker or prime of prime relationship. The capital requirement and regulatory requirements here varies across country.
Second, since in the forex world there is no centralized exchange. You would need to convince each individual ECN, market maker or big bank, exc. exc. to allow you to provide liquidity to them. Each firm will have its own set of requirements.