The Trading Game by Alfred Slote (1992, Paperback, Reprint)

What's So Special About Ace 459?

Andy Harris's fabulous baseball card collection, left to him by his father, is the envy of every baseball fan in town. Still, Andy would rather play ball than collect cards. He's got a natural talent for the game, like his grandfather, pro ball player Jim Harris. And Jim Harris, Ace 459, is the one card Andy would give anything to own -- he'd even trade his priceless 1952 Micky Mantle card.

Then Grandpa comes to town for a visit and offers to coach Andy's ragtag team. For the first time, Andy and his friends really look good, really feel like a team. But Grampa's rules for playing the game contradict everything Andy believes about friendship and good sportsmanship. And Andy begins to wonder if Ace 459 is such a hero after all.