DM's Guild is the officially supported way to sell homebrew D&D content using official WotC materials like settings, characters, etc.

And yes, there are professional Dungeon Masters that people can hire to run games for them. Since D&D is in the midst of a major revival, people who are interested but have never played before are looking for someone to run a game for them in much the same way that some people get paid to teach football or baseball or what have you.

In my experience people who have played D&D for a long time think this is weird or even sketchy, to accept money to play a game with someone. In my opinion, DMing is a skill, and there's nothing inherently wrong with being paid to employ it. As long as food and shelter still cost money, I'm not gonna fault anyone for trying to get paid.