I want to pull periodic reports via Google Seller API. Is there a way, to login into Seller account without having to go via regular OAuth redirection ?
It would make no sense to try to use a service account to access AdEx, which is owned by a user account. You need to obtain a refresh token for the user account, store it somewhere, then use it to obtain an access token whenever you need to access the API. This answer gives the steps to follow How do I authorise an app (web or installed) without user intervention? (canonical ?)