Can I buy an iPhone off craigslist to use for Straight Talk?

I have a straight talk account and want to get an iPhone. Do I need an unlocked phone or does any phone work? If I bought a phone off cragslist for verizon would it work? I need all the details possible b/c I'm tired of all the other phones I've gotten from straight talk.

You will need an…

UNLOCKED

GSM

ATT OR TMOBILE IPHONE

you can always ignore me… And buy a used CDMA Iphone that was from Verizon, Sprint, Virgin Mobile, Cricket, or Metro, or Altell… Lmao… See what happens… Ill be here when you get back… Ill tell you "sorry, no fix, wanna cookie?" and laugh