While YouTube channels can have multiple email accounts associated with them, they will always have one primary owner, and this account is the one that has access to everything. When you buy YouTube channels from someone, you need to make sure that they sign the primary owner to be you, so that you're in charge of everything. This request can take up to 24 hours for YouTube to approve. Once you've been approved as the primary owner, you need to make sure that you delete all other channels linked to it, so you retain complete control.