Selling app to businesses.

You could give them the source code (project files) and let them submit the app themselves or you could modify it with their graphics and submit the app under your account.

The problem with you keeping the app under your account is that you have to maintain that for as long as the contract is good. You could charge an annual fee to cover that.

If you give them the source code, you could have a contact so they can't sell it to others.

There might be a different way that I don't know about, maybe someone else will chime in.

These business can't restrict an iOS app to only their customers (except for a very small number of customers on an annual support basis). It has to be available in the App store for everybody. That's Apple's way for apps on iOS devices.

If that's ok, what you can do is to have each customer to enroll as an iOS developer themselves. You can charge a nice annual consulting fee for helping them to do this this as a service, since it's unlikely they can figure out how to do it themselves. Then customize branded apps and submit these apps for the clients using their developer accounts (not yours). Make sure each customer (or their legal department) understands all the App store contracts and legal agreements, and gives you a contract authorizing you to do the stuff you do for them.