I've bought a ton of used lenses from Ebay, almost never had a problem.
I follow the philosophy that if you buy camera gear from someone who can't take a good photograph, you're taking a risk (clearly not a photographer), and I look for a good clear description which says things like "no fungus, no scratches, clear optics" etc, so that you have maximum comeback if the item is not as described.
So far, I've only had one bad experience and I'm 95% sure that was the post offices fault, not the sellers.
Saying that, there's something to be said for a physical shop so you can try before you buy and you know where you can take the lens back in the event of a problem. I have used
www.srsmicrosystems.co.uk extensively, their shop in Watford is an fantastic place to go and look at/play with/chat/buy camera gear. The owner, Chris, is a great guy and only seems to employ people who like cameras.