Almost all my equipment is second-hand, except for my Sigma 70-200 2.8 lens purchased new online from 42nd Street Photo out of New York. Some of these camera dealers also sell used equipment that has been cleaned and inspected. I got some items on eBay and Craigslist. My 40D I bought from my photographer friend.
Any Canon or Canon-compatible lens labeled with EF (their mounting system) will fit any line of Canon camera bodies, whether they are a full-frame body (like the 5D, 5D Mark II, 6D, etc.) or crop-sensor body (Rebel line, and 30D, 40D, 50D, 60D, etc.). However, the EF-S mount lenses, which are typically “kit” lenses you can buy bundled with a Rebel, will ONLY fit the crop-sensor bodies. This is due to a longer back part of the lens where it mounts to the camera. Thus, they are much cheaper, but much lower in quality. You can get those second-hand for around $100. If you plan on eventually buying a full-frame body, it’s best to invest in an EF lens. They are just better all around in quality and versatility. They can be quite expensive, from several hundred to $2,000 +, depending on which lens, the type of glass, and the brand (Canon brand lenses are usually more expensive than their other-brand counterparts.) Sigma and Tamron are two great brands.
Another thing to keep in mind with lenses- on any crop-sensor body, any lens you put on it you will have to multiply the focal length by 1.6 to get the more accurate focal length (when compared to a full-frame body). For instance, I have a 35mm f/2.0 prime (non-zooming) lens. On my full-frame 5D Mark II, the focal length is actually 35mm which is a wide-angle (I believe 50mm is considered “normal” angle) so objects tend to be a bit distorted especially at the edges of the frame. When you shoot with a wide-angle lens, you can be standing quite close to the subject and get more in view than you would in real life. In contrast, when I put the 35mm lens on my Rebel or 40D, I have to multiply 35 x 1.6 to get what the focal length will appear to be, which is 56mm. Therefore, on a crop-sensor body, 35mm is not really a wide-angle, or does not behave as such.