Comment by joan_kode

Dark web marketplaces are not exactly "a system where nobody trusts anyone". They are pseudonymous, as opposed to completely anonymous, and sellers absolutely build reputation and some degree of trust. Otherwise nothing would work - the problems you point out would prevent any business at all. It's theoretically possible for a hitman to build reputation in such an environment. However, it would have to be comparatively slow and high-stakes, which I suspect is the main problem and tips the scale towards scamming.