Personally, I would be somewhat cautious about buying expensive lens off ebay (even though I buy other things). Also, there is no need to buy a "digital" lens -- just research the lens you are most interested in first (I would suggest the canon slr lens form on dpreview.com) and how it works with your camera. (As a practical matter, newer Canon lens are all fine -- some fairly old models might have autofocus and/or metering limitations given that I know this is the case with Nikons).
For used glass, I would suggest KEH.Com -- they have a great reputation and their condition ratings on the used glass tend to be fairly conservative.
For new glass, I am a fan of B&H photo although Adorama has also treated me very well. BuyDig also has a following.
I would also be certain you understand what lens you are buying. There is a big difference between a 70-200 f4 and a 70-200 f2.8. Not that one is always a better choice but, in practice, they are pretty different in weight and price (and, of course, one is a full stop faster). Brands other than Canon (i.e., Tokina, Tamron, etc.) might offer a good value -- and might not -- it all depends on the model in question. Be certain you understand the differences between a USA and a grey market lens -- while the lenses are the same the warranties are not. Not the buying grey market is a bad thing, just that you should understand the limitations.