I am not a big fan of buying used. Especially buying consumer grade cameras and/or lenses. It is OK and a good way to get some equipment but you must know either the person you are buying from or how to test the camera/lens very throughly.
Buying use pro gear is usually a good deal as it is so much more durable. It is also worth reparing if necessary.
A good place to get used cameras is KEH. They give you a good return policy. B&H and Adorama are good sources, too.
Be careful on Craigslist.
Personally I would never be a buyer at 70% on a Rebel. Especially three models back. More like 50% is in order. And, IMHO, no more than 50% on EF-S lenses. They are just built too flimsy.
The "L" line is quite different though. Some actually go up in value on the used market. And 1 series cameras seem to hold value well.
You should check out the Canon Online Store (assuming you are in the U.S.), the refurbished camera section.
They are out of stock of the T3i right now, but get it in from time to time. You can put yourself on a list to be notified when stock is available. They sometimes have sales there, too.
It looks like T3i usually come with an EF-S 18-55 IS II lens. If you are interested in other lenses to complement it, there's a refurbished lens section on there on the website, too.
Refurbished are often just demo models or similar that have seen very little actual use and I believe they all come with a full warranty.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.