Of course you need to sell your product but as a builder you can do it yourself. It's not your specialty so you likely will be worse at it than a dedicated person and will have less time for actual building.
The key is that builders can exist without salesmen. But salesmen without builders have nothing to sell.
In civilizations, people used to primarily be farmers but as food production increased, this gave rise to the specialization or division of labor. In the nature of an evolutionary competition of companies, those that specialize into builders and sellers will do better than builders that try to do both, therefore the former paradigm will win out.
I am against some specializations getting paid per unit of work regardless of the market value of their product while other specializations get paid a cut of the market value.
I am against positions of power which allow people who don't produce anything to decide how much other people who do actually produce something get paid.
Of course you need to sell your product but as a builder you can do it yourself. It's not your specialty so you likely will be worse at it than a dedicated person and will have less time for actual building.
The key is that builders can exist without salesmen. But salesmen without builders have nothing to sell.