There are a few reputable sites which provide a "rating" to the condition of the camera.
So first... go check the Canon REFURBISHED section of their online store. This has the advantage of not only being checked out by Canon... it has the added advantage of actually coming with a warranty (usually a 1 year warranty for most cameras in their refurbished section.)
You can find that here: http://shop.usa.canon.com/shop/en/catalog/cameras/refurbished-eos-digital-slr-cameras
Other sites (I do not personally endorse these sites nor have I actually bought any used gear from them) include:
KEH.com (they specialize in the resale of used camera gear and they "rate" the condition of the gear)
The B&H Photo Video store has a "Used" department. bhphotovideo.com and find the link at the top for "Used Dept."
Adorama also has a "Used" deparment... go to adorama.com and find their "Used" link at the top of the page.
You may also look for repuatable local "real" camera stores. Most "real" camera stores (not big box stores that happen to sell cameras) will have a "used" department. These have the advantage of being local so you can personally check out the gear.
At the bottom of the list are eBay (which sometimes offers purchase protection if you follow their instructions) and Craigslist. The risk here is that you're buying direct from a seller who may be mis-representing the gear or it's condition (regardless of whether they do it deliberately... or accidentally simply because they don't know enough about gear to provide an accurate assessment.)
I have to agree with and reiterate what the previous posts have recommended, can only expand on that a bit and add a few more suggestions.
Assuming you are in the U.S., first start at the Canon online store and look at the refurbished cameras there.... Often the gear you find there is very lightly used and comes with a good factory warranty. There are sales there from time to time, too.
Regarding the recommended retailers, over the years I have purchased many items from KEH.com, Adorama.com, and bhphotovideo.com. KEH is a used equipment specialist who sells a little bit of new stuff too. Adorama and B&H are the two biggest NYC retailers and have large used equip. departments. All of them rate condition very accurately, offer liberal exchange policies and warranties in the event of a problem. They are quite reliable and purchasing from them is very low risk, but items might not be discounted as much as they are elsewhere (at more risky sites).
There are others... on Amazon.com you will find both new and used items. Used items are usually sold by partners, who are reliable but you do have to know what you are looking at and have a feel for fair prices for used items when shopping there. Prices set by the partners can be all over the map for some items, ranging from great deals to prices higher than the same item brand new. So be sure to shop around. Some offer return policies and a few even short warranties on used items. Amazon.com keeps a pretty close eye on their partners, so there is less risk buying there than some other places.
Locally, you might look on your Craigslist. This is riskier, no warranties typically. But prices can be great (again, shop around carefully)... And you can meet with the seller and check the item out before handing over your money (you have to know how the item works and be able to quickly put it through it's paces to confirm it's good). Also only meet CL sellers in very safe place, such as a bank lobby or inside a busy coffee shop. Most CL deals require cash and sellers showing up with a pocket full of money have been robbed and worse. So just be careful.
eBay is another possibility.... But not without its risks. The feedback program helps you identify reliable sellers. And some items come with short warranty and liberal return policies. But many do not. Pricing can be good... But in some cases is terrible! Bidding can be fun... Or very frustrating. Again, you have to know what you are looking at.... And check it out thoroughly as soon as it's received, while still within any right to return time window.
A couple forums.... Fredmiranda.com and photography-on-the.net both have classified sections where you can connect with private sellers and get some pretty good deals, too. Long term forum participants are very reliable, but this isn't without some risk, since items are mostly sold as is and without any warranty. You also need to zero in on what you want and have a feel for current fair values.
BTW: You'll have much better luck if you look for a "Canon" camera, rather than a "Cannon". There actually are "Cannon" cameras out there... Cheap plastic knockoffs that, some of which are labelled "Olympia" (rather than Olympus) or "Nikkon" (rather than "Nikon"), etc. Not recommended!
I rarely buy used because I find a lot of sellers think their stuff is made out of gold.
I am not going to pay $2100 for a used 70-200mm f2.8 if the new one is $2200. It just is not worth it to me.
I buy a lot of new stuff from B&H and Adorama and have struck up a relationship with a few of the sales staff.
Helen Oster at Adorama is great.
What I am trying to say is make sure you know the new price and service that comes with it before you decide used.
I have a friend who is retired now and he bought virtually everything he has used from KEH. He even buys their "Ugly" and
"Bargin" rated stuff and seems quite happy with it.
And example he got a 1D Mk II for $200 buck as "Ugly". It gave up the ghost shortly after he got it. He had CPS replace the shutter for $250. $450 bucks for a good working 1D Mk II. That was several years ago and the camera is still shooting today.
Caveat emptor, let the buyer beware.