Factory unlocked iPhones and gsn for craigslist?

I'm a noob at phones, and I've noticed that with cell phones there's a thing called gsn that you need to look for. Stolen phones would have bad esn. So my first question is: do factory unlocked phones have gsn?
my second question is, I've seen some pretty cheap factory unlocked iPhones on craigslist, some seem too good to be true, I'm scared that the phone would be fully functional when I use it, then when I bring it home it bricks on me. What should I look for in an iPhone when meeting up with someone from craigslist?

Added (1). MYBAD IT'S ESN

No idea what gsn is?
gsm carriers are the ones that use sim cards it thats what you meant.
when meeting to buy a iphone try and do it at the carrier so you can ask if its stolen or blocked, if not pop in your sim make a call and send a text.

If it is a GSM based iPhone, I would check the IMEI number with an online IMEI checker like www.iphoneox.com This will tell you if it is factory unlocked. If there's an application called Cydia on the iPHone that most likely indicates it was jailbroken. If you go to Settings, General, About and you look at the MODEM verison and it says 6.15.00 then that means it is unlocked using the iPAD baseband. This makes it difficult to restore in iTunes and normally to get it to work with iTunes you have to downgrade this baseband… So a few things to look for.