Can i use a used iphone on straight talk?

If i buy an iphone off of craigslist can i use it on straight talk if i buy their sim card? Or does it have to be a new one

First make sure it's NOT a Verizon iPhone, they don't have sim cards.

You can use it on Straight Talk if it's an ATT branded iPhone or if it's unlocked.

Go here: http://wiki.howardforums.com/index.php/Straight_Talk_iPhone

If you need more help.

Go here: http://www.howardforums.com/forumdisplay.php/551-Straight-Talk and look around and you'll find all the help you'll need.

Go here for iphone info:
http://www.everymac.com/systems/apple/iphone/iphone-faq/iphone-sim-card-troubleshooting-upgrade-from-original-to-iphone-3g.html