Some are:
Shopping
Mall Japan
Rinkya
Anime Deputy
(Sato)
Celga

These people bid on yahoo japan auctions for you.
 I highly reccomend Shopping Mall Japan (AKA
AnimeChaos).  Next would be Rinkya, then
animedeputy, and I highly discourage you from
using celga.  

Both ShoppingMall Japan and Rinkya have what is
called "live bidding".  This allows you to place
your bid in real time through their automated
system.  AnimeDeputy and Celga have a real person
bidding on your behalf, which can lead to error. 
Good luck!