Really it comes down to the price/value, just like everything else. The amount of wear that a lens has is based on how popular it is. The nice thing about LensAuthority is that they check out the lens before selling them for decenteredness, resolution etc. It saves a bit of time checking it out.
I purchased a 100-400L from them during a Black Friday sale for 950. It was rated 8-8.5. The lens body was in excellent shape and the front/rear elements were clean. The lenshood is a bit looser than a new lens' because of the wear, and the bottom of the lens foot was scuffed up (all of it was disclosed in the lens description), so I got what I thought I was going to get.
All things being equal, I'd rather get Canon refurbed units for the same price (better warranty, etc.), but not all lenses are available there. I almost got my MP-E 65 from LensAuthority (they sold out during a sale), and I got the 100-400L and I'm happy with what I got for the price I got it.
Small scratches in the coating don't affect image quality significantly but it does affect resale value. It might be worth waiting for a 8.5 or another unit that has cleaner glass for that reason alone.