It is actually possible to check out the items that are for sale on the Black Market Auction House on every single server without even logging in to your WoW account due to a new website that brings you up-to-date information. The website is found at http://www.wowbmah.eu/, and it gets updated a few times each day. It includes all the European realms, and currently there is no US counterpart.
It is possible to check the BMAH for a single realm by using a specific URL such as http://www.wowbmah.eu/realm.php?fetch=EU-Ravencrest.
This service will surely be put to good use by mount collectors who are willing to use a character transfer to another server to buy a rare mount, and now they won't even have to manually check all the realms anymore.