When my wife's iPhone was stolen a few weeks ago, I put my phone number in on the lock screen. The third thief to buy it was dumb enough to call me and ask for the unlock code.
We had the phone back in a week, the guy that called me escaped a felony charge for receiving stolen goods, not so for the thieves ahead of him.
You can get them unlocked easily for 40-50 bucks however that circumvents the stolen property issue.
When it comes to Apple products they do a good job of making them useless if stolen but there are shops out there that will unlock them for you.
I won't buy stolen merchandise. Bad karma.