You don't get it : many shops/companies do
auction on ebay because it make advertising
for their business.
For shops, auctioning on ebay can only be a
marginal part of their business.

And yes, in case you didn't noticed, they make
money from all auctions. It's not because
someone is selling other goods on their own
site that it will changing anything to the
money ebay is making. And that certainly won't
push people to sell more on ebay and less on
their sites.

As for the best place... it's certainly because
of something they are doing : it's just because
they were the first one to do such things : it's
harder for another site, like Yahoo, to start
as people sell on the site where there is the
most visitors. If enough people start using 
another auction site, then many will stop selling 
on ebay.