Baseball great Alex Rodriguez and his business partner, Marc Lore, are investing in a new company aiming to be a “stock market’’ for professional athletes, allowing fans to buy and sell players whose values rise and fall based on their performances on the field. Mojo, as the new business is called, plans to debut by the end of the year, with football as its first sport. The company has raised $75 million in a funding round led by Joshua Kushner’s venture firm Thrive Capital. Tiger Global Management also participated.