I don't understand how a market maker would operate?
The necessity for a market maker seems to be build on a false premise that there are not always users willing to buy or sell. This is never true, there are always users that want to buy from or sell to you, just not for the price that you like.
So you don't like the price...
How can a market maker change that?
How can a market maker buy or sell bitcoins for the price that you like (which is always a better price than the users are willing to offer)?
Let's say he always puts up the most competitive bids and asks.
From what I see when the price goes up he would end up selling btc cheaper as he can buy them back, going broke. Correct?
Yes, there would always be buyers or sellers at unfavorable prices, and this means you might have trouble selling or buying without a lot of slippage from the present market price for larger orders. The role of the market maker is to provide liquidity or volume closer to current market prices than would otherwise be possible, so you can sell or buy with more volume and less slippage in price.
This is just a basic sketch, but market makers make their money in the spread. For example, a market maker (MM) might be buying bitcoins for $100 and selling them for $102. The MM is making $2 for every coin, and with heavy volume this can add up quickly. This doesn't mean that the MM is always making money. If suddenly the MM can't find any buyers for the coins bought at $100, the MM might have to sell them at a lower price. In that situation, the MM would take the loss on those coins, but set up shop with a spread around a lower price (say buying at $96 and selling at $98). With skill and proper risk management, the MM can avoid heavy losses and turn a profit in the long run. And the MM is providing a helpful service for other market participants by providing liquidity.
Here's the Investopedia entry for market making, which has a little video that runs through the basics.
http://www.investopedia.com/terms/m/marketmaker.asp