Find the answer to your question
What are the different accounts and keys that I need to build an eBay application?
Here are some of the terms that will help you understand the differences between a developer account, keys and tokens:
To start the development here is what you need to do:
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