A platform with two groups of users who transact with each other, where your job is to bring both sides on at the same time and take a cut of every match.
Pierre Omidyar's auction site connected buyers and sellers of anything, took a small final-value fee, and let the network do the rest. By the late 1990s eBay was profitable while most of its peers were burning cash, because the marketplace itself owned no inventory. The same shape (two sides, a fee per match, near-zero inventory risk) became the template every later marketplace tried to copy.