I've bought a 135mm L, a 10-22mm and a 1.4x II secondhand from my local dealer, all with a 12 month warranty and I've been overjoyed with both lenses, not used the 1.4x much, but it's useful as a keeper.
Personally, I've always been suspicious of eBay and the like, I can drive down to my local shop, handle the lens, compare with a new one, compare with alternatives, they would even take a purchase back if I changed my mind after a few days. I pay about halfway between new and a good low eBay price, but I'm confident of my purchase.
SUPPORT YOUR LOCAL SHOP !!!! Before online retailers remove the option of handling and trying in person. Both can coexist in the market, there might be a premium initially, but many local shops have price matches and offer that little bit extra that many online retailers can't