We are trying to test with a test reseller account and have created an additional location for that under Account & Settings / Business Information / Locations. We cannot find the location_id (format XXXXXXXXXXXXX) in the dashboard to use in the API calls where it is required. We can only find the location_id under the application object in the main developer account dashboard.
The test reseller location_id isn’t exposed in the Developer Dashboard. You’ll need to use the access token that was generated for the account to call ListLocations to get the location_id. 
Yes, thank you; I had recently found the ListLocations API. So, can you confirm that in a real-life reseller customer account, that when they go to the Locations section in their Dashboard they will be able to add locations and also see the 13 digit location_id identifier without us (or them) needing to do an API call?
This brings up another topic: test coverage using the sandbox versus a production account. We really seek a real-life test environment to get a good confidence in our implementation. For example, it would be desirable to have a real payment link page where the tester can enter a bogus PAN like repeating “4242” to make the payment versus seeing a page saying that you are in the sandbox mode and then click here to simulate a payment going through. Also, we need to do a real-life test of the OAuth flow with the code, access token and refresh token. Do most developers set up “real” accounts to do this type of testing and then use prepaid cards to issue the test payments and then the discount rate charge is part of the cost of development?
Unfortunately, the ability to test production links with sandbox values isn’t available. You can definitely create another account to test OAuth. Please note that any payment that’s processed in production you will need to use a active card.
So, can you confirm that in a real-life reseller customer account, that when they go to the Locations section in their Dashboard they will be able to add locations and also see the 13 digit location_id identifier without us (or them) needing to do an API call?
If your using OAuth why would you ask the seller to get their location_id? Ideally with OAuth you would never have to ask them to get either location_id or merchant_id.
Is that a “yes” or a “no”?
The seller will only see the location_id in the URL. Its not shown anywhere in the Seller Dashboard.