Shopping around has a real cost, and that alone explains differing prices for one item

Classical economics assumed buyers instantly know every seller's price, but George Stigler pointed out that finding out actually costs time and effort, so prices for an identical good routinely vary across a market simply because searching every seller isn't worth it past a certain point. Stigler's 1961 paper treated information itself as something with a cost and a value to acquire, launching the economics of search that later explained everything from price dispersion to job-hunting behaviour.

— George J. Stigler, The Economics of Information — Journal of Political Economy, Vol. 69, No. 3, 1961