Like everything else, this all comes down to "Buyer/Seller Beware." You have to do your homework.
eBay seems like a great place to sell lenses, but not such a good place to buy them.
I'm amused at the retailers who list products on eBay for more than they are selling them on their websites. I guess they've figured out that there are a certain percentage of eBay buyers who don't bother checking prices. Like others, I have often noticed used items going for more on eBay than the refurbished store price (which comes with a warranty and the assurance that Canon has brought the product up to specs.)
I have had the best success on eBay with accessories, especially lighting accessories and camera bags (always used, never new). I imagine that's because both are items that people buy, use for awhile and then either don't need anymore or decide they need something different.
I've bought from the refurbished store. Like anywhere else, you need to compare prices and decide what is and isn't a good deal. My main complaint with the refurb store is the randomness of their stock. Some lenses show up quickly and frequently, while others never appear. For example, I have a hard time believing that Canon never gets any 200mm f2.8 primes in for refurbishing. I mean, it's a lens that's been out for 16 years, but none ever get returned to stores? Yet, the 100mm Macro L showed up on the site less than a year after it was released. Still, it's a fun place to window shop and see what they have.
My personal philosophy is, if the margin is small, I'm willing to pay a premium for new. Second choice is refurbished. I've never bought a used lens or camera and would need a significant discount to convince me to do so. But that's just me.
The great thing is that in the internet age we can instantly compare prices and even have resources like Canonpricewatch.com to do most of the work for us.