I mostly buy new, only rarely buy used.
A couple years ago I bought a used EF-S 15-85mm lens from a reputable individual. It turned out to be a bad purchase. The lens was not very sharp. I don't think there was a fault with it, but that all these lenses are not that sharp. This despite the fantastic reviews it garnered when it first came out. Because it was a private sale, I was not able to return it. I sold it instead, at a loss.
When you buy used from a private seller, you limit your options if you change your mind.
I suspect some gear is sold because the owner is not happy with it. In some cases, there is a fault with the item. I once had a Canon 100-400mm lens that occasionally would lock up when tracking birds. It only happened when pointed up and zoomed to 400mm. Once locked, it would not focus until zoomed in. That restored its function, but of course resulted in missed shots. The lens was sent in to Canon but they said there was nothing wrong with it. But there was. I was able to return it to the dealer and exchange it for another copy of the lens. What happened to the first one? I'm sure it got sold to someone else! Hopefully the current owner does not use it for tracking birds.
Occasionally you read stories here on POTN about someone who bought a used item that was not as described, for instance a body with a much higher shutter count than purported.
I bought a Canon 100mm macro new and hated it because it lacked IS. There was nothing wrong with it, it just didn't have a feature I needed. Returned it the next day to the dealer and exchanged it for the 100mm L IS version and am very happy with it.
If a good deal on a used item comes along, I might go for it, but I'm usually happier with new purchases.