So, let's say you moderate a subreddit about Thinkpad laptops, or trading cards, or anything else. As the moderator, you can put links to vendors in the sidebar. You can auto hide any comment naming a competitor, or only prune positive reviews of a competing vendor. I'm sure a lot of mods get paid to do this, but plenty are also just the owner of a vendor who represents themselves on Reddit as a neutral third party.

Here's a nyt article on the wallstreetbets mods trying to get paid out in a movie deal