I'll try explain the idea more concisely:

• user wants to promote own website
• user creates ads (small banners and gifs) like these and hosts them on an instance of the software through their website
  • the server-side implementation would have an API to fetch the URL of the advertisements from to embed to the website (just simple image files or gifs)
• user asks other people (friends, others in the fediverse) to save their website on these peoples' own lists of websites that they are willing to host the ads for
  • people would host based off of similar content, interesting topics, and general goodwill as opposed to exposure (as very few personal websites get constant exposure to large audiences) and revenue (as this would be a willing move)
• the client-side implementation of those hosting other websites' ads would randomly pick a URL from the user's own list (similar to picking a random URL from a webring), use the API (something like /get_ad?) to retrieve the URL of a random ad from the promoting user and display that on their website
• "automatic" was a bad word choice, I'll change it now
• this wouldn't solve a problem, just automate the functions of webrings by giving every user their own decentralised "webring" (the list of websites) and displaying user-curated ads (probably at the bottom of the page where most banners are) as opposed to randomly picking from a webring
• those using personal websites would be the users, while visitors would be the audience.

should've made the wording more clearer in the post, my bad I guess. and to clarify, this is just an concept I thought about though and I don't actually have plans to develop this. (I've also edited the post with my final opinion on the subject.)