Hi there and welcome to iMore.
You don't.
If it's stolen and has been reported and deactivated, it's no better than an expensive paperweight.
Sorry to be the bearer of bad news but buying a stolen and deactivated iPhone is never going to work.
What you do is go to the Police with details of the seller and pass it all over to them, there's nothing more you can do for the phone and it might help catch the person responsible for selling stolen goods, perhaps.