Most sports games get overlooked by collectors because they are dirt-common and get outdated every year. Some sell for good money though. I was just trying to come up with a list of these sports games that are worth quite a bit. Here's what I've come up with so far:

NES:
Jimmy Connors Tennis

Genesis:
Mutant League Hockey
Mutant League Football
Madden '93 Championship Edition

SNES:
Chavez II
Mountain Bike Rally

PS1:
NBA 2 Ball demo (would get good money if anyone could find a copy to sell)

GBA:
Baseball Advance

GC:
NCAA Basketball 2k3 (I think this is the one)

These are the only ones I can think of off the top of my head. Any others?