Early juvenile sports stories, especially those written from the 40's through to the 70's are always worth scouting. I've found over the years that the genre desirability is roughly in the following order of interest: Baseball, Baseball and Baseball then Golf, Martial Arts, Surfing, Ice Hockey, Basketball and Football. (I admit to an American bias here, I have handled and sold very few Soccer books.)
Genre collectors are still interested in condition as well as content, but overall scarcity of the item rules the day. A few years ago I had a rather thin, unimpressive juvenile bio of a baseball player which I listed online at a modest price which sold within 6 hours. The book was only in very good condition, the author was unremarkable, but it turned out that it was the only bio ever written about that particular player. A fact that the second, third and fourth people who called about the book were very quick to point out.