Never look there. I mainly look at Ebay, KEH, and rangefinder forum.
One thing is for sure, Canon has more lens options. There are allot of 2 consumer and 2 pro (F2.8 & F4 or weather sealed) lenses at different focal lengths. Ad that with Bjrichus comment on market share. Many canon users are moving up or down depending on needs. Nikon just doesn't have a large line that is in the middle ground.
I don't think you can make conclusions about satisfaction based on used items for sale on a site. I know many Canon users who love their glass and tout it as better than any other companys'. If someone is confidant in the tools they use than that gives them confidence to be able to make great images no matter the brand.