Definetly check out FredMiranda. I have bought and sold several lenses there. Do some research on the going prices for the lens you want and then be ready to react quick to a good deal. The do come up on there frequently but as others have mentioned they also go quick. I'll second the reccomendation for the IS lenses. I have a Nikon so they call it VR, but it really is amazing for low light shooting. I eneded up giving about $1400 for my 70-200/2.8 VR Nikor. The Canon is just within $100 of that same price typically.Both Canon and Nikon's f/4 in the same focal length are very good lenses, but I don't find to be fast enough for concert shooting.