I want to let my customers buy ads do display on my platform. I might not be using the right buzzwords to search for a good one.
I would start with a simple payment platform like Paddle or Gumroad and simply manually copy-pasting the add for the given amount of time (e.g. one month).
Unless you plan of having hundreds of avertisers on your site, it should work fine. Or are you trying to build some sort of AdSense/BuySellAds competitor?