You can seek a career in any line, offering anyone your services and products. You and I can decide to work together or not. We can produce, trade, buy and sell, along with millions of others. The main point is that no deal is made unless the people involved go into it voluntarily. They cooperate to make it happen.
Libertarians support the free market for two reasons.
First, it's right. No interference People have the right to deal with each other peacefully and honestly. If, by our honest work, you and I produce different things of value, we have the right to trade what we have produced with each other. We have the right to make any deal which suits us both. All of us have more opportunities to do productive work, doing what we like best.