The online site and store Justballs is designed to appeal to a specific vertical market and exemplifies a growing trend among Web merchants. Justballs offers 1,300 items, some of them ball-related accessories but most falling into the category of actual balls ranging from footballs to 'squeeze balls.' The site was founded by Jim and E-Ping Nie Medalia, a husband-and-wife team getting out of the Web-design business to start their own shopping site. The Medalias recognized that most of the Web's growth potential is in markets less mature than the books and music focused on by Amazon.com. They