Want It Now is a free want-ad service in which eBay buyers post notices for items. Sellers review these listings and respond. So, for example, if you're looking for an album by harpist Dorothy Ashby and cannot find it on eBay, you can post a Want It Now ad which might read:
I am searching for: Afro-Harping, an album by Dorothy Ashby.
Describe it in more detail: I'm looking for the compact disk in good condition. My price range is $10 to $15.
You can even post a picture of the item if you have one. Don't put your email address in the listing as eBay will connect any potential sellers with you via your eBay account.