(Reuters) - A new trading cards business launched by sports retailer Fanatics has raised $350 million in fresh capital from new and existing investors at a valuation of $10.4 billion, according to a person familiar with the matter and documents reviewed by Reuters. The latest financing round was led by existing investors Silver Lake and Insight Partners, the source said. Endeavor, the owner of the Ultimate Fighting Championship (UFC), also invested in the round, the source added. The size of the early-stage funding round highlights the boom in the trading cards business, which has helped drive up valuations for companies like Fanatics. Earlier this year, the new company, called Fanatics Trading Cards, secured long-term exclusive rights deals with several major sports leagues, including Major League Baseball (MLB), National Football League Players Association (NFLPA) and the National Basketball Association (NBA). Fanatics holds a controlling 80% stake in the business, with the players leagues and associations and other investors owning the remainder of the company. As part of the funding round, the leagues and players associations have raised their stakes by about $1.4 billion, the source said. The fresh capital will be used to create a sports platform to expand the existing trading cards offering for collectors, according to the source. The new venture is looking to leverage Fanatics’ existing fan base of loyal customers to build out a sports commerce business, which operates across gaming, sports betting, media business units and retail stores. Josh Luber, co-founder of sneaker marketplace StockX, will join Fanatics Trading Cards as co-founder and chief vision officer. The parent company Fanatics, which is backed by SoftBank Group and was valued at $18 billion in a recent funding round, operates online stores and sells licensed products of over 300 teams and brands, including Manchester United, National Football League and NASCAR, among others. Reporting by Krystal Hu in New York and Manya Saini in Bengaluru; Editing by Anil D’Silva Our Standards: The Thomson Reuters Trust Principles.