When selliing a expensive lens online, your seller reputation is extremely important. If you do not have a history, then you will not get a top price. Thats true on ebay, or Fred Miranda.
However, if you live in a heavily populated area, you can list on Fred Miranda (A nominal fee is involved) and specify that it can be inspected and picked up locally. Thats likely the only way you will sell it.
You can list it in your local Craigslist, and you might be suprised. There are lots of people looking for such a lens and would like to see it and take a few images before they buy. Just do not invite anyone out to your home, arrange to meet in a safe location like a Starbucks. Also don't succomb to listing it at a inflated price just to see if anyone will bite. Serious buyers will mark you as a scammer and will not call even if you lower the price to something reasonable. Do your research first, set a fair price, and stick to it. Have some documentation showing that you are the owner, and hopefully that it was cleaned / inspected at Canon recently with a clean bill of health. This investment will pay off.
Expect to get $3800-$4400 depending on the condition and your selling reputation.
Here is a popular database of priceshttp://www.jcolwell.ca/photography/lens$db/index.htm
hope that helps.