You can own the software, the accounts, and the documentation on paper and still find that only one person can deploy it, restore it, reconcile the data, or change a critical workflow. Sometimes that person is the seller. Sometimes it is one employee, or the developer who originally built it.
Tell me what the system was, where the handoff broke, and what evidence you would ask for before close if you ran the deal again.
If enough of you answer, I will write up the patterns here. No names unless you want yours used.