Accounts and keys required for building an application

Answer:

Here are some of the terms that will help you understand the differences between a developer account, keys and tokens:

• A "developer". This is you: a real person who does not exist as an eBay user but has two sets of keys (DeveloperID, ApplicationID and CertID) - one for the Sandbox and one for Production.
• A sandbox user, who could be a buyer or seller and who is the "client" of the developer. This is an imaginary person set up as a user on the sandbox.ebay site. You need one token which allows the "developer" above to represent this client on the sandbox via the application.
• An eBay Marketplace user, again a buyer or seller who is a real client of the developer (the person for whom you are writing the software). This user is actually doing live transactions on eBay. You need a new token which allows the "developer" above to represent this real eBay user via the application.
• A Token is an ID which authorizes your application to access eBay data on a specific user's behalf. User tokens are only needed when using eBay APIs that require user authentication.
  
  Thus to develop using the Sandbox and Production, you need two sets of developer keys and two tokens at minimum, and there will be at least three "entities".

To start the development here is what you need to do:

• Join the eBay Developers Program, create a developer account and get Sandbox & Production keys
• Use the eBay Sandbox User Registration to create Sandbox Users
• Generate a token for the users using the Authentication Token Tool or the steps outlined in the documentation
• Write your application and test in Sandbox
• Register production users on eBay
• Generate tokens for Production users using the Authentication Token Tool or the steps outlined in the documentation Deploy the application in production.

Additional Resources:

Get Auth`n`Auth tokens: https://developer.ebay.com/api-docs/static/get_auth_n_auth_tokens.html
Get OAuth access tokens: https://developer.ebay.com/api-docs/static/oauth-tokens.html
Create the eBay API keysets: https://developer.ebay.com/api-docs/static/gs_create-the-ebay-api-keysets.html
Follow best practices: https://developer.ebay.com/api-docs/static/gs_follow-best-practices.html