TOKYO (Reuters) – Shares in Yahoo Japan Corp slumped 10.2 percent to a near four-month low on Friday after it said it will buy mobile network operator eAccess from Japanese wireless carrier Softbank Corp for 324 billion yen ($3.17 billion). Yahoo is a content provider. What are they picking up spectrum for? a Tokyo-based senior trader at a foreign bank said. This is a very much non-core business. It doesnt make much rational sense for them. Yahoo Japan shares were the fifth-most traded on the main board, while SoftBank, down 2.7 percent at 7,604 yen, was the most-traded. …