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This is for the case where you, as a contract developer, have developed an application for your client. Assume the application is now on their system, they have tried it with their (or your) sandbox keys, and now they want to use their own production keys with the app installed on their system.
In this case, the client (end-user) should create their own set of production keys. They should create a user account at developer.ebay.com. They (or you) should log into the account and create a set of production keys here
This will give them three strings :
Most applications store these strings in one configuration file (or store in a database). They should replace the sandbox keyset with their prod keyset in this config file.
Sometimes the developer (you) gets a prod keyset and gives the keyset to the end user. This will work if you will "always" be around to provide support for the app which you've given to the user. But if you will be out of the picture, the user should get his own keyset.
If the end user needs more than 5,000 daily calls, you (or the user) will need to apply for an App Check at http://developer.ebay.com/support/developersupport/app_check/Default.aspx
When making the application, please provide the production keyset that you'd like to receive higher call limits.