As the blazing Arizona sun signals the start of spring, a different kind of migration takes place in the Valley. While 15 Major League Baseball teams flock to the Phoenix area for the Cactus League, a separate breed of enthusiasts also descends on the region. These are the devoted baseball card collectors and traders, eager to capitalize on the annual surge of interest in their niche market fueled by the return of America’s pastime. With all the Cactus League stadiums conveniently located within…