I hired a guy years ago for a project who was an "Oracle expert." I would have put that guy on any project involving databases as while he clearly knew Oracle inside and out, I have no doubt his knowledge would have transferred in some form to other platforms. Can you make a living being a "Postgres expert"? That's hard to say. Anymore if I were hiring someone I might want them to have more know-how that just the guts of a database, IE: I'd want them to know something about the systems on top of the database so that they can make stronger recommendations.