Some sources I worked with:
FM - There's a bit of blind trust dealing with people there, but I've had nothing but an excellent experience buying from other owners, regardless if it's an old $20 film camera lens, or a $2500 DSLR. The people selling there tend to know their stuff and take the time to describe the condition of the item and any issues it has or had, if any. Prices are reasonable and even great. Used items generally don't have a warranty anymore (if originally bought more than a year ago), but cameras and lenses tend to be reliable enough that it really doesn't matter to me. Still requires exercising some common sense though as the site is the target of scams from time to time. If the deal sounds too good to be true or the method of payment / shipping sounds fishy, give it a pass.
Craigslist - Great that you can check out the item yourself, but I'd recommend it only if you really know what to look for. Most things will be priced higher than it's worth so you'll have to go through the hassle of negotiating the price. Then there's dealing with non-communicative sellers, scheduling where to meet, maybe driving out 30 minutes to meet somebody, waiting 30 minutes because the seller is late / delayed, etc.
eBay - Much riskier. You'll need to know how to check, or guess, the condition of the camera through pictures and the item description, which are usually lacking. Sometimes, problems are intentionally hidden or you'll have to read between the lines on how it's described. Not too bad if the seller offers a return period, but more often than not sellers do not accept returns. Even then I've had success forcing through returns simply because it's completely different than how it was described (like, not working at all). Ebay's buyer protection has worked pretty well for me. On the positive side, I've gotten some really great deals from there though.
Some sellers on eBay also has some great deals on brand new stuff, but they tend to be import / grey market models whose warranty may not be honored in the US.
Canon refurbished - Cheaper than retail, and essentially brand new. At least a one-month warranty, but many refurbished cameras come with up to a 1-year limited warranty. Won't come with a retail box, which strangely I sort of care about. Sales tax and shipping will also eat into any savings you may be getting.