Transfer IOS apps is a very easy process. All process takes around 1 minute, once you have the developer's Apple id & team from the receiver developer.
How to find your Apple Team ID & Agent Apple ID
Apple ID= The Email the developer used to register in iTunes
Team ID= Can be found here when you login with your apple account
Still having issues? Check this tutorial video.
Q: How I transfer apps in iTunes platform?
A: Simple click on transfer app from your main account
Q: Why this isn't working for me?
A: Basically, there are 2 main reasons on why you can't transfer apps. First, one of the things listed here is not completed. Second reason can be that your client didn't setup payments data yet, and can't accept the app.
Q: How can my client accept the app?
A: First ,he will get a notification of a pending app. He needs to login to his iTunes, go to Agreements, Tax and Banking, and confirm. This looks like this