While the work of APs is considered the “primary” market, another key player, market makers, is needed in the “secondary” market, for example on exchanges, where most of the trading is done. Market makers build on the role APs fill by buying ETF shares when others want to sell them, and vice versa. If prices get out of whack, they can earn a profit by trading to nudge them back in line. In some cases, market makers also play the role of the AP.