I asked the same question on reddit a while back so I'll give you my experience and how the sale went. I found someone selling a GoPro Hero 4 Black for $290 shipped. Not to mention, it appeared to me that GoPro's lose a TON of value in general when sold used because everyone is too scared to buy them not knowing what the person prior did to it. $210 off the sticker price (at the time of sale!) for one brand new? What? Ok, so then I saw that there was a small crack in the upper left hand corner of the GoPro's front faceplate. This means that somebody dropped the thing somewhere along the way. But the seller was honest about this fact and insisted it still worked fine.
It seemed too good to be true, although that model wasn't refurbished. What redditors told me is that GoPro customer service is usually really good and they will take care of it even if you're not the original owner. Of course the problem with "refurbished" anything is that you're always left asking "what does that mean?" Did the lense have a tiny scratch on it and needed replaced? Was it as simple as a button being broken? Or something more sinister. You never know. So it's a catch-22.
Do you have an opportunity to ask the seller to record and upload a video? When I bought mine, I asked the seller to do this and write their forum/seller name on a piece of paper just to verify its legitimacy. They did so, and this was good enough for me even though I could have gotten stuck with a dud. And that was it, I have my GP and am happy with it.
Used GoPros aren't the devil, and they can take a beating and still survive. I think they're made to do so
If I were to fix that front plate (I think it's a $5 item?!) no one would ever know it had been dropped or damaged cosmetically! But see, that's also an example of a possible "refurb" situation.
BTW. Certified refurbished cameras
are tested by GoPro
and include standard 1-year warranty. So, if you can get that verified as well, and when it was refurbed, this would be of interest!