You can never totally trust used gear. Most of it is fine in my experience, but a small number of items have hidden flaws. It might take days or months for you to discover the issue. An expensive lens I had once had AF that froze intermittently when shooting up. Canon couldn't reproduce the issue, but the fault continued. I finally returned it to the dealer and exchanged it for another copy (which worked fine). What happens to faulty items like that? They end up back on the shelf at the dealer, or as a refurb item at Canon.
Yes, Canon refurb gear is used gear.
Several times in different forum threads I have seen people asking about odd faults with equipment that they bought used.
I used to buy a fair bit of used gear, some very expensive, and have generally had good luck with it. But there are no guarantees. These days used gear seems to be overpriced, so now I only buy new.