If you want actual platforms to sell through then:
https://acquire.com/ - Very serious buyers, listed companies are manually verified, choose this if you have decent revenue
https://www.buysellstartups.com/ - Listings for micro startups
https://www.microns.io/- Another for small startups
I’d highly recommend getting the app audited if you’re planning on selling, will go a long way in terms of buyer’s confidence in the app’s quality. I think @georgecollier offers something like this if I’m not mistaken.