I want to buy iPhone 6 off craigslist. It is listed under 'unlocked'?

I want to buy iPhone 6 off craigslist. It is listed under 'unlocked'?

Added (1). I was curious if I could pop my current sim card out of my iPhone, and use it in the new one?

Yes you can. You can use any provider with it.

I suggest that if you are using straight talk or tmobile pre paid. That you use a Tmobile iphone 6. This means you can use LTE on them. What I mean is that if you used a tmobile sim card it wouldnt get LTE on an atandt iphone 6 since atandt runs on different frequencies.