Talmudic law and the medieval law codes developed from it protected consumers from many types of deceptive practices.
A seller charging more than fair market value can be forced to rebate the difference, or even to cancel the sale.
Jewish law provides for government intervention in the markets for goods and services through price controls, limits on profits, and perhaps even allowing cartels to operate.
Shortchanging a customer in weight or measure, even mere possession of defective weights or measures, is a punishable offense -- and a moral failing.
The Torah forbids fraud in pricing and in weights and measures. Prophets and sages rail against those who violate these laws.