I have been trying to sell my Canon EF 70-200mm F/4.0 L IS USM Lens for ages now, but no one seems to want it. Could it be because the f2.8 is a better lens and people would rather pay the extra and get that rather that the f4?
Since you did not tell us anything about how you are 'trying', I can't really give you a complete answer. Now, it is a good lens and has a nice price-point too.
Things to consider:
- Price. You need a to offer a discount compared to the lowest price from a reputable source. Check BHPhoto and Adorama if you are in the US, otherwise the local equivalent.
- Location: Craigslist, Kijiji and eBay are all places with lots of visitors.
- Description: Make sure the description of the lens is accurate in your ad. Don't complicate things but make it easy to find, specifying the range, full model name, etc.
- Patience: It takes me between 1 and 6 months to get a buyer sometimes on the listing sites. With eBay things are more pressing but it can take 2 or 3 tries.
Relative to that lens itself, it may be that this model being the middle of the line makes it less desired. The non-IS model is much cheaper and the F2.8 model is not that much more expensive. There is also competition from the Sigma 70-200mm F2.8 (OS and not) as well which is a very good lens and is reasonably priced.
It is possible you simply haven't found the correct audience, or it could be that your price is too high.
I personally would be happy with a f/4 version because for what I tend to shoot, I don't necessarily need the f/2.8 version, and I'd prefer to save money and weight.
If you really want to sell it quick, you could go ahead and sell to a reputable dealer such as KEH. They aren't going to pay as much as a direct buyer might because they need to make profit, but they will pay pretty well.
At the moment, (assuming "Excellent", or 80-89% condition) it looks like they'd pay about $550.
I would certainly prefer the f/2.8, but another issue may be that many people are hesitant about buying a used lens from an unknown source. When I switched from Canon to Nikon, I think I sold most of my stuff on Craigslist (which works out well because people can meet you to check out the stuff) and Ebay (but I have a high feedback rating).