There is a rating on their condition. 

If you get an iPhone, I would get it checked out at your local apple store to make sure there are no issues in case any future problems arise. 

But make sure you are happy with it prior to the return date. 

If you have the money I personally wouldn't go pre-owned. I've heard some horror stories on the community. I'm sure they are a small percentage, but it's a risk that I am not comfortable with.