Did you ever solve this?
I got this to work myself. There are a couple of things which stop it working:
First - if you're in sandbox mode you need to sign in with a sandbox user account. I had issues with this because it had created passwords automatically and they never seemed to work properly.
The issue seemed to be that because you were already signed in or previously signed in with another ebay account, the browser remembered this and got in the way of the sign in process. It was easier to use a different browser entirely for the Authorisation sign-in, and make sure all cache to do with ebay was cleared - copy the link on the RuName list called "Your branded eBay Production/Sandbox Sign In (OAuth) - see all", and paste it into the other browser, taking care not to include the 'see less' on the end. NB - if you've had a few failed attempts at this its better to create a new RuName by clicking "Add eBay Redirect URL" at the top.
On the sign in page - if you get the username or something wrong, or you have to click 'change user' first, it won't work, as the URL will change and won't be correct. Hence making sure your browser is cleared of ebay cache first.
Make sure all of the "Your auth accepted URL" box is filled in, with a link to your app / page where you are doing the authentication work.
The documentation is very badly laid out on this - almost everything requires clicking a link which opens a new article explaining something. Sort it out ebay!
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