If you buy from Amazon Renewed, for example, you have no idea exactly what you'll get. You could get a pristine unit without a scratch on it with a battery that has barely 10 cycles, or you could get a well worn (with plenty of micro scratches and minor nicks) with a battery that's been cycled 500+ times. It's kind of a crap-shoot when it comes to buying refurbished.
As someone who takes pride in keeping my phones absolutely perfect and even micro-scratch free, it's too much of a risk.