I realise this is an old question but just in case this helps someone else.

When Getting the OAuth access tokens you need to:

Set grant_type to client_credentials. Set scope to the URL-encoded space-separated list of the scopes needed for the interfaces you call with the access token.

So in your example set scope to "https%3A%2F%2Fapi.ebay.com%2Foauth%2Fapi_scope%2Fbuy.marketing"

Then use the returned OAuth code with your query