What should I know before buying a second hand iphone 4?

So I've found a good deal for an iphone 4 but the thing I'm worried about obviously is getting scammed. I'm pretty sure it's not a China product but what I'm worried is things like if the iphone 4 is unlockable or not, the IMEI is blacklisted(do i have to worry about this too?). What other factors should I include when buying this?

Added (1). ** i'm actually going to see it for myself

Ask for the serial number so that you can check that the phone is 1: legit 2: not stolen 3: not a phone that was replaced for being faulty. If you can, pickup the phone yourself so that you can see for yourself that it's real.

Make sure it is not jailbroken if you do not want a jailbroken iphone. Also make sure you ask for the serial number and check on the apple website to see if it is legitimate. I would give you a link, but surprisingly apples systems are down for maintenance.

Hope this helps