How do i find the email address used as an icloud ID for an icloud locked phone?
Take it back to the dealer. See if they have the contact details. If not, get your money back, they should have checked before even buying it from the previous owner.

The entire email address is never exposed, so you won't be able to access it. 

Even if not stolen, if the original owner doesn't remove it from their iCloud account, there's no way to bypass the Activation Lock.