Slinger Bag, the U.S.-based start-up that is selling the innovative tennis ball launcher by the same name, started trading on a special over-the-counter stock market on Dec. 24, reaching a market capitalization of $24.2 million. It has moved up from the OTC Pink exchange to the OTCQB Venture Market, reserved for better early-stage and developing U.S. and foreign companies. To be eligible, they must be current in their reporting and undergo an annual verification and management certification process. They are also required to meet a minimum bid price and may not be in bankruptcy. Slinger Bag says it has accumulated more than $200 million in retail value through the numerous distribution agreements that it has secured since last spring under the management of Mike Ballardie, former CEO of Prince and head of Wilson for the EMEA region.