Easy, really:

1. Most Japanese sellers don't understand 
English, just like you don't understand Japanese. 
Usually they can barely read romaji, much less 
English. Believe it or not, although it's always 
been made out that "every japanese person knows 
english as well as they know japanese" is just a 
flat-out lie. Most Japanese know enough english 
to read simple stuff, but not much more than 
that. Really good evidence of this can be seen at:
http://www.engrish.com
People in Tokyo overall seem to have better 
English skills, but have you tried Hokkaido 
lately?

2. There are a lot of Japanese out there who 
still hate the Americans and usually 
any "friendly" countries associated with America. 
I know we all think that everything is hunky-dory 
now, but the USA did drop bombs on Japan that 
killed thousands of people, and we also were at 
war not but 55 years ago. I know my grandparents 
(who fought in WWII) think I am crazy for 
liking "Those awful Japs". Well, in Japan, there 
are still some very hard feelings towards the USA 
still. Sometimes it comes out in the form that 
they just won't sell to the US, for ANY price. 
Period.

3. Foreign anime fans have a really bad 
reputation in Japan for creating fan sites, 
putting licensed pictures up on sites without 
permission, copying doujinshi without permission, 
etc. Haven't you ever noticed the total LACK 
of "commercial" art on Japanese sites? This is 
because it's illegal and not appropriate in 
Japan. A lot of Japanese are horrified at this, 
and refuse to sell to foreign fans because they 
don't want it splashed up for the world to see on 
every web page in the universe. They also don't 
trust that the cels won't be copied and sold, 
therefore destroying the legitimacy of the cel 
market.

I could go on and on.