Satoshi is for all practical purposes not a billionaire. The assets have not and perhaps will not ever be liquidated.
This is speculation. He could theoretically have borrowed money with his bitcoin wallet as collateral in a private transaction.
How do we know that he didn't sell the private keys?
I think purchasing private keys is an unlikely transaction because it would be impossible for the purchaser to know how many copies of the keys existed before the transaction occurred. It could theoretically happen.
You could buy an obligation of him to transfer you the bitcoin on request, like other regular contracts. Kind of like a swap agreement. Of course, some level of trust is required.