There are two sites that openly publicize this 
type of service...(I know one other has been 
mentioned here...but the name isn't coming to 
mind)

1. Sato (only thing...he's shut down his service 
for an undetermined amount of time) Still, if you 
REALLY want it, and since it's not a bid, he 
would probably help you out. ^_^

http://www.d1.dion.ne.jp/~aico_co/framepage2.htm

2. Celga - this one's probably your best bet. 
They advertise themselves as a middleman for 
Yahoo Japan auctions, but I'm sure they would 
help you buy something off a personal Japanese 
site as well. 

http://www.geocities.com/jpanime_com/

Also, I know there are quite a few people on this 
forum who are fairly proficient in Japanese. They 
might not be willing to bid on an auction for 
you, but I bet someone here would help you to 
communicate with the Japanese dealer. 

Good luck...

MeganD.