It is possible, but....
I do not think he would "sell" his wallet.dat to some secret buyer, and then that buyer would not move all coins once, to ensure their control.
If what you say did indeed happen (theoretically), I would think Satoshi would of passed the wallet.dat to someone, but only if he would be dead soon.
So in that event, he is dead now and his coins are in the hands of a friend/his spouse or his child(ren).
Selling a wallet.dat (and not moving the coins) is too risky for everyone otherwise.
Do you really think Satoshi would trust someone that much?
You know Satoshi was very secretive and made sure it would be difficult if not impossimble to ever be found.
A question, why should he trust another person instead to trust himself? Is this a basic concept of the whole bitcoin 'system' ... don't trust no one, trust only yourself (aka your wallet).
I remember that satoshi said (here in the forum) that we should never delete an 'address' or worst an entire wallet.dat , because it is not recommended.
Exactly, and that is why I don't think he would trust anyone else with all that wealth.
If you will trust a third part service all the whole (or entire) bitcoin concept will 'go'. I think it is better to pay in the traditional way, instead to give the private key(s) to another person.
I really dont understand. Why do you want to sell your coins with wallet.dat?
Do you think will it worth more than its balance? If so , why people give extra money for your wallet when there are lots of issues like another copy of your wallet.
Maybe because someone doesn't have a connection internet and he can give face to face the copy of the wallet.dat , but as you said ... it is really an insecure way to give 'btc' to another person. Because who knows if you have deleted that private keys? You can still own the wallet.dat.