Displaying paid content equally amongst all users

I'm building an app (mobile and web) in which users can sell products and services. You could say it's some kind of "small ads" for this purpose. Using the application will be entirely free if you wish, with some advertising to monetize it somehow in the future, but I want to sell options to make your content displayed first for a fee.

I don't know yet if the content to be displayed will be based on the connected user's query and filter, that shouldn't be a problem to set once I get there, but what's bugging me is this: How can I display paid content equally, fairly amongst all the "matching" paid elements? I don't want users complaining that the content they paid to be seen is never showed (quite understandable). Is there an established way to split the "views" so they will all be displayed approximately equally?

Maybe there's already an answer for that, but I don't quite know how to "search" for it, and with my choice of wording, I didn't find something helpful (everything was about AdWord, Facebook advertisement, ranking in search engine, etc).

Thank you!