Buying used is a good way of saving money while increasing one's gear available for use. However, there are some important factors to consider.
Gear sold by a major photo gear retailer will often provide a guarantee of functionality (or, at the very least, in the condition listed). And if you are unsatisfied with your purchase, you can often return the gear without hassle. Because of the minimal risk involved, you are likely to pay a little more for your used gear when purchased from trusted retailers.
Buying your used gear from a personal sale involves more risk, but you're more likely to get a really good bargain if the gear performs as advertised. Unfortunately, if you are dissatisfied with your transaction for any reason, getting your money back (if even possible) may prove problematic.
Neither method for buying used gear is necessarily better than the other. However, both methods have their benefits and drawbacks, and it is important to understand the risk/reward ratio before deciding on which purchasing platform is best suited for your needs.
B&H Photo - Check daily as B&H occasionally has 10-rated (or email-for-condition) reconditioned/refurbished Canon lenses.
Adorama - The online used Canon lens inventory is generally strong here.
Cameta Camera - Located in Amityville, NY and typically has used Canon Lenses.
KEH - Again, used Canon lenses are often available at this retailer.
Midwest Photo Exchange - Used Canon lenses are often mixed with the new lens listings at this retailer.
FredMiranda.com - One of the most active camera equipment buy/sell forums on the net and rich in used Canon EF lenses. Reasonable deals on used Canon lenses sell extremely fast here - sometimes in minutes.
SportsShooters.com - Many used Canon EF lenses show up in the classifieds section.
NaturePhotographers.net - Login required. A buy/sell forum that usually has many used Canon EF lenses.
Used Gear: Personal Sales via Major Retailer
eBay - Used Canon EF lenses bring a premium at eBay (good for sellers), but the selection is usually large. I placed it in a category on its own because while it may be a peer-to-peer transaction, eBay typically provides a money back guarantee ensuring you receive the item as described in the auction. Be sure to check auction details carefully to see if your transaction is backed by eBay's coverage.