Watch out for the older Sigma lenses. If you are buying used, be aware that some of them will not work on a Canon DSLR due to the faulty Reverse Engineering that Sigma did back then. You cannot control the aperture and they give error 99. There were updated versions of some that work fine, but you need to be able to test it.
Sigma charged me over $100 several years ago to update my 105mm Macro, but they could not update my other 3 Sigma lenses since they were out of production. I've been wary of Sigma since then.