Hi Dan,
I think you will have to be aware of the mount of a used lens from a compatability point of view. I understand Nikon mount, but not Canon, so perhaps Sassan or other Canon user can offer advice, as I believe Canon have changed their mount several times.
Here is a link that may offer some clues.
http://photonotes.org/articles/eos-manual-lenses/
Other than the mount compatability I should not expect any probs. I am using an arsenal of 20 year old M/F lenses on my Nikon.
Don