I want to sell my app, how do i do this?

You will need to enroll in the Apple Developer Program in order to release your app on the App Store. Have a look at this for more information:

https://developer.apple.com/programs/

There is an annual $99 membership fee that includes the cost of hosting the apps on the store. You don't have to spend any extra money on top of that in order to sell your app. If it's a really good app, Apple may put it on the Featured page, which doesn't cost you anything and gives you some extra publicity.

Are you saying you want to sell an app/project to a developer or are you asking next steps on getting your own app in the store?

If this is about becoming a developer, see the program link bob provided, then see https://itunespartner.apple.com/en/apps/overview

Marketing your app is, however, all about budget and time. If you lack one, leverage the other.

Google is your friend on that topic. Example:

h t t p : / / apptamin.com/blog/app-marketing-strategy/

Hi Bob,

I do not want to promote my app to the general public on the App Store, I want to sell the app to someone who has the time and money to develop it further and has a budget to market it properly.

Can I avoid the $99 charge?

As the app is not in the App Store potential buyers can not use it to see its potential, is this a catch 22 situation?

Of course you can sell the source code and the rights to the app to another private party. The buyer will take it from there, and you won't have to enroll. Once you sell the app, it's the other person's responsibility to market it. And if your agreement with the buyer includes some sort of royalty payment, it could end up working out very well for you. 🙂