This isn't really what market makers like JS do. While they do make profit from the spread between prices they buy and sell at, the key difference is that they

This isn't really what market makers like JS do. While they do make profit from the spread between prices they buy and sell at, the key difference is that they will buy from seller A and sell to buyer B at different times. For the duration between the two trades, they either own a positive or negative amount of whatever the good is, so they are at risk of losing money if the price changes. The spread pays them for taking on the risk and providing liquidity (allowing people to buy and sell any time even if there's no matching counterparty).