The first recorded baseball game was played in Hoboken, NJ in 1846 between the NYC Knickerbocker Base Ball Club and the New York Baseball Club (the Knickerbockers lost). By 1858, the National Association of Base Ball Players was formed, consisting of around 30 small teams. After the Civil War, the Cincinnati Red Stockings became the first professional fully-paid team.
Over the next hundred years, a number of leagues emerged, including the National Association, American League, National League, Federal League, and Continental (items from defunct leagues are highly sought after today).
Baseball cards are widely collected today. The earliest incarnations (1840s) were cardboard-backed photos of local players and teams, which came in cabinet card size and the smaller carte-de-visite size. Commercial baseball cards emerged in the late 1860s when a sporting goods dealer called Peck & Snyder printed cards with an ad on one side and a baseball player on the other. Within a few years, cards began appearing in packages of Old Judge tobacco and other tobacco, candy and chewing gum brands.
Other promotional items such as baseball programs became popular in the 1890s. Advertisers bought space on these score cards, and cards from famous games (e.g. the World Series) are highly sought-after. Other collectible promotional items include gloves, statuettes, plaques, posters, and pins.
And of course, there's a huge market for authentic baseball memorabilia linked to specific players, including greats such as Babe Ruth, Ty Cobb, Joe DiMaggio, Hank Aaron, Mickey Mantle, Jackie Robinson, Ted Williams, and Willie Mays. They produced plenty of signed baseballs, bats, gloves, photos, and other autographs. But be careful, as fake sports memorabilia is a lucrative business.