* Retailers in Japan struggling with weak consumer spending (Adds background, company comment): TOKYO (Reuters) - U.S. retailer Walmart Inc (WMT.N) has decided to sell Japanese supermarket chain Seiyu and has already approached major retailers and private equity funds, the Nikkei business daily reported on Thursday. If realised, the sale could amount to around 300 billion to 500 billion yen ($2.7 billion to $4.5 billion), the Nikkei said, without citing its sources.