Some used Canon lenses are hard to find

A lot of people don't like the kit lens and get rid of it to buy something like a 24-70 or 10-22 which they then love and hang on to, bout it really.

The owners actually like using these lenses and don't want to sell them. If I had a 18-55 I'd probably give it to Goodwill whereas both the 10-20 USM and 24-70L would have a permanent home in my bag. And, yes, the economy has slowly been inching up so people tend to hang onto things they like. I know a few guys whom sold most of their gear during hard times a couple years back and are now trying to replace it all albeit at higher prices.

It's probably because people who have them don't want to give them up. I doubt it has anything to do with hanging onto lenses until the price goes up.

Kit lenses are all over the place precisely because they're kit lenses. Most beginners start out with them, and many of them eventually move on to either a super-zoom vacationer lens or something else. Thus the used market gets flooded with kit lenses nobody wants.

Although you asked this question in connection with Canon, I'm sure it's much the same with Nikon, Sony, and other brands that appeal to consumers.

I've got feet in so many camera families that I am starting to look like a centipede, but Craig's idea that this is nearly universal is true. Even early AF lenses for the Maxxum and other such lenses that still fit currently sold cameras can be hard to find and expensive by old lens standards when you do.

With the increase in 4/3 cameras and cameras like the Canons being able to use older MF lenses (especially attractive to video shooters), even "obsolete" lenses like Exakta, M42 and other lenses have seen price increases in the face of a old camera market that is otherwise still declining.

Its obvious. If you have an EF-S 10-22, why would you sell it? What would you replace it with? Ditto for the 24-70/2.8.

Unless you are giving up photography , or in the case of the 10-22 switching from APS-C to FF, you'll keep them. There's no "upgrade path" for either lens.

The Canon 500 mm F4 is difficult to find right now - for anything less than $6k (used)...I am also seeing fewer 600mm F4s for sale (used). Try finding a used 1DsIII - these are commanding high prices on the buy/sell markets as well (upwards of $6k).
rdc/nyc

Lenses vary in quality even among L series offerings. Once you find one that performs well - why would you sell it uness moving on or a definate change in shooting circumstances?

First, as others have pointed out, not many lenses like this get sold unless someone is getting out of photography or is switching systems.

That being said, I haven't seen that much of a drop-off in availability. Fewer lenses are being sold via dealers like KEH (where sellers get the worst price) and ebay (where there is a lot of hassle), but there are still lots of lenses available for local sale (like Craigslist and such).

I'm actually selling Canon 10-22, just can't find a buyer... LOL
But I am not selling on Ebay for obvious reasons, just on Craigslist.... takes time I guess.