— Why do I need it if there are a couple of other gems with OmniAuth strategies for eBay?
eBay has two different authorization methods: Auth’n’auth and OAuth. Technically, they both use the OAuth2 protocol.
What is the difference? Access tokens!
With Auth’n’auth you will get a single token which you can use to access only old eBay XML APIs (Trading API, etc.) With OAuth, you will get a pair of access and refresh tokens that can be used to access new eBay REST APIs (Buy API, Sell API, etc.)
However, you can use new OAuth tokens to access old APIs, too—by providing an access token in HTTP header
X-EBAY-API-IAF-TOKEN. This is documented on eBay developer program website: Using OAuth with the eBay traditional APIs.
If you plan to use new APIs, you are welcome to use this gem together with ebay_api client gem for REST APIs.
For old APIs, you can look at ebay_request gem (you can configure it to use OAuth tokens).