The answer is almost never the vendor. What to insist on when you resell, and what it costs you later when you do not.
This is the single question that determines whether a white-label relationship is a partnership or a hostage situation, and most agencies do not ask it until they are trying to leave.
Vendor owns the account. Convenient at setup, catastrophic at exit. The historical performance data, the conversion history that the bidding algorithms have learned from, and sometimes the remarketing audiences all belong to someone else. Leaving means starting from zero.
You own the account. The account sits in your MCC, the vendor gets delegated access. Exit is a permissions change. This is what we do and what you should insist on generally.
The client owns the account. The most defensible arrangement and the one clients increasingly ask for. It also means a client can fire you and keep everything, which is a real commercial consideration — but a client who wants to leave will leave regardless.
Conversion history is the part people underestimate. Smart bidding strategies get better with accumulated data, and a fresh account with no history is measurably worse for weeks. That is not an abstraction — it is a real performance cost paid at the exact moment a client is already unsettled by a vendor change.
Then there is the tag container, the analytics property, the Search Console verification, and the Business Manager assets. Any of these sitting with a departing vendor turns a transition into a project.
Accounts in your MCC or the client’s. Analytics properties in an organization you control. Tag containers under your account with the vendor as a user. And a written statement of what happens to access on termination, agreed before you start, not negotiated when you are already unhappy.
Send the shape of what you are carrying — how many accounts, which channels, where it hurts. We will come back with specifics.