I saw many american on Ebay that aren't willing
to sell outside of USA...

Fact is, you can't force someone to sell you
something if he doesn't want to. Whatever
you think his reasons are justified or not.

The case is even worth with Japan : Most japanese
on Yahoo Japon doesn't speak english. Even if
they were willing to send to another country, they
are not able to understand what you wrote them
and they can't tell you how to pay, where to
send the money and so on...

Also, USA is not the whole world. Just because
you are used to do things in one way doesn't
mean that it's everywhere the same. You may use a 
lot personal checks but in some other countries
it's not used at all.
Japanese people, if they accept to sell to you,
will usualy want that you pay them by wire 
transfer. Because that's the easiest and nearly 
only way for them.
Are you willing to send money that way ? If not, 
that's your problem and not their....

IMHO (and I have a nice feedback on Yahoo Japan),
if you can write to the seller in understandable 
japanese, provide clear information about how to 
ship internationaly and agree to pay with the 
method they ask you to, very few refuse to ship to
another country...

Cheers,
Yann Stettler