That's true, if everyone is willing to use labour cost as the limit of price - otherwise profit would recreate the system of capital domination. It seems like a hard bargain to maintain if a currency substitute like labour vouchers are used.
It can only happen if unlimited concentration of capital is possible in the first place. In property right systems based on use (i.e. where society refuses to protect other property claims), this is not the case.
I understand that, but I believe that a market based system allows a capitalist system to be bootstrapped from within anarchy. If I'm able to trade for profit within the market, say in a voucher system, I can use those vouchers to purchase security from other less ideological people and bootstrap the coercive relationship between profit and violence. Maybe free market anarchists have an answer to this, but I'd hope that a revolutionary society could do better than repeating the inhumane parts of capitalism that originate from the market and could recreate it wholesale.
Note that freed-market anarchism is just one end of the more broader free-market left libertarian spectrum, which does not intrinsically preclude governments and law enforcement.
With that in mind, consider what would happen today if you were to claim some random empty house as your own, posted a sign to that effect outside, and started to shoot any "trespassers". That would be murder - and the law would come down on you. Same thing would happen in this hypothetical society if someone tried to do what you describe.
(The issue of how a fully anarchist society would deal with murder in the first place is a separate and complicated one. I can't really speak for them; my own take is that anarchism is a kind of political asymptote - an unachievable utopia that the realistically-possible ideal society would trend towards as its cultural underpinnings evolve to make it more viable. In this day and age, something like Bookchin's libertarian municipalism appears to be empirically viable.)
