Why would you want to? That pretty clearly means it's stolen property. You would be abetting a criminal who has chosen to make a living stealing someone's most personal (and expensive!) possessions. You'd also technically be breaking the law yourself. 2 people close to me have had these $700 plus devices stolen from them, one which was BRAND NEW. And let me tell you, it is a huge gut punch especially when you don't make a lot of money. Don't help these awful people by buying from them.
I'm aware all the sellers of these frequently give a ridiculous story like "oh, I received this from someone I know and they forgot to unlock it. But instead of calling them and having them unlock it I'm just going to take the big loss by selling it this way." They are obviously lying.
The fact that eBay and other marketplaces even allow these devices to be sold (openly!) is disgusting.
To answer you, they generally can't ever be activated or even used as a non-cellular device unless they are returned to their rightful owner and unlocked with the password. The reason there's a market is because people buy them to part out. There's a whole industry of people who enable this disgusting behavior by breaking them down and reselling them. They all have very bad karma.