The differences between eBay and Yahoo are numerous. The most important difference can be characterized by saying that Yahoo is a Seller's site and eBay is a Buyer's site. Most of that has to do with protection and accountability. Both sites are worlds apart when it comes with dealing with Buyers and Sellers and is, in my opinion, the biggest reason why the sites will not be able to have any kind of significant partnership in the future. In the end it all comes down to a Japanese person's willingness to do international transactions, and that won't be changing anytime soon.