There's a LOT of items that sell on ebay - used - for prices very close to new, sometimes even more than new. It's incredible.

I've seen quite a few used Ultegra cassettes recently and most of them are selling within a few dollars of what you can get them for new if you do a tiny bit of internet shopping.

As for your experience with 9 speed stuff, it could be this phenomenon (ebay buyers paying "new" prices for used stuff) or it could be the market forces others have described. My personal opinion is that it is the ebay overpaying phenomenon. Very few "good deals" on stuff like that on ebay.