I'm not sure if there is such a thing as a truly free market, but I do believe in order to approach that, everybody needs to be paying their own bills.
For example, let's say a factory pollutes a river in order to manufacture widgets which then get sold at a profit. Is the company paying the cost of cleaning up the pollution? If so, then that cost should get put into the price of the widget. If the company is not paying for the cleanup of the river, then either the people who rely on the river for food or water are "paying" with reduced health and enjoyment of said river, or the government pays the cleanup bill at taxpayer expense. The government might make a regulation that requires company to pay its bill by cleaning up the river it messed up. The regulation might get complicated to cover for all contingencies. Then freedom minded people complain about government overreach, and they have a point.
The best solution is that the company is ethical and pays all its bills because that's the right thing to do. Then you might have something closer to a free market.
Thanks and very good point. There are perhaps hidden costs to certain activities that should be taken into account as you point out.