My opinion:
Those who have good names that serious domainers will start a bidding war over, go to the domain auction sites, such as Sedo.
The "not so good" domains, that serious domainers would have no real interest in, go to ebay, where either someone who sees potential in the name or a noob pick them up.
Obviously, more people have the last type of domain I mentioned, so more domains are put for sale on ebay.
ALSO! There are names that make you go "WOW! That sold?!" on Sedo and some great names on ebay. But, for the most part, what I posted seems to be what's up.