I haven't bought a used optic, but I sold a few.
For optics with reputation as being more durable (Aimpoint, EoTech, Trijicon red dots), I'd be more open to buying used. For me, I'd like to at least know the seller and that they aren't the use and abuse type.
For higher dollar scopes (say $1000 and up), I'd likely buy new unless I knew the previous owner and I was able to test it out. Usually not worth the savings, at least in my mind. Well, maybe I'd give one a try if it was ridiculously cheap, but for stuff like optics, I'd like the knowledge that it wasn't abused.
With regard to selling optics, I already assume I'm gonna take a hit and have to sell below what I feel it's worth. For me, the price to buy a used optics would have to such that the savings offsets the risks of buying used.