I bought two used Canon L lenses on ebay. But I did my homework first.
First, the seller has to have an outstanding rating, especially if farther than a 2-3 hour drive from where I live. I bought an 80-200 f2.8L from a Japanese retailer that had a rating over 10,000. I had the lens in my hands 2 days later...and I'm in New England, USA. The EF 200 f2.8L ii I bought was from a seller with a rating less than 500, but had sold a number of higher-priced items, including some photography equipment, and had a 100% rating. Note, too, that some very reputable, highly regarded photography equipment retailers such as
Adorama sell on ebay, too.
The second thing I look for is condition. Pristine or very nearly so is what I am looking for. Dents, dings, scratches, no thanks. Well worn contacts at the rear of the lens, no thanks. (Look VERY carefully at pix of the contacts!). Any visible or noted flaws in the optics, no thanks. Missing original packaging (retail or part of a kit, I don't care), no thanks. I'm very picky when it comes to spending 100s of my dollars! For what it's worth, I bought my 30D and 2 EF-S lenses on ebay when I first moved up to a DSLR. I was just as picky then as now.
I've also sold the EF-S lenses and a couple of Ls I didn't use enough to justify on ebay. I sold them in the same condition as I bought them...pristine...original packaging, and even the original store receipt for lenses I bought at
B&H Photo.