Richfield-based Best Buy, which has seen CD sales slip in recent years, has entered the world of online music. Through a new partnership with RealNetworks, Best Buy is the first major retailer to offer a music download service to its customers. Best Buy will resell RealNetworks Rhapsody service, which requires users to pay a $9.95 monthly subscription fee plus 79 cents for any songs they download and burn to CD. The service has the backing of the music industry, which is trying to wipe out illegal downloads. Scott Young is Best Buy's vice president of digital entertainment. He says the service will appeal to customers new to the online music environment.