By Dan Weil Prime retailers will be able to offer free, quick delivery and free returns, using payment information stored on Amazon. Amazon (AMZN) – Get Amazon.com, Inc. Report has often been accused of discriminating against third-party sellers on its site. But now it’s launching a new service designed to help both other retailers and its own Prime customers. The service is called Buy with Prime. Retailers will be able offer some of the same benefits to Prime customers that Amazon does, such as free one- or two-day delivery and free returns. Retailers will be able to use Amazon’s fulfillment …