When buying doujinshi from Japan using a shopping service, why do sites like Toranoana and MelonBooks refuse to sell to well-known or big proxy companies?
And also why is it when Yahoo!Japan Auction sellers find out one of their auction items will end up overseas refuse to ship internationally?

(Btw, does anyone know a list of any other online doujinshi stores that don't sell to big shopping proxy companies?)

Speculation: Probably an informal Agreement to prevent them ending up on international News easily.