I got my 500 f4 G VR used from a friend of mine and I would probably do it again. Earlier I bought a 200-400 f4G VR used and that was o.k. too.
If you don't know the seller personally - like I did in case of the 500 - and you don't have the chance to go there getting hands on the lens and test it, I suggest to prefer retraders that offer a return policy (professionals in most cases). The lense might be ab it more expensive but you pay the difference as a kind of insurance fee for being able to return the item within 14 or 30 days and that's o.k.
My entire equipment (bodies, lenses, flash(es), angle viewer, ...) I bouhgt used, except my big Gitzo with Gimbal and my 500PF and the only reason I bought those new was that they weren't available used at the time I wanted/needed them.
I would say the experience was positive in more than 90% of the cases.
When I buy used equipment I usually try to find out what it has been used for in the past. Reason is that I once got the chance to look at the gear of a pro photographer following the WRC races around the world and what I saw was a shock of a lifetime. So, whenever the gear was used in outdoor race sport, for action near or on the ocean, I prefer to keep my fingers off even if the price looks tempting. Also a past in sports press photography can be a risk.