2 firms complete in a market by setting prices for homogenous goods. See "Kruse, J. B., Rassenti, S., Reynolds, S. S., & Smith, V. L. (1994). Bertrand-Edgeworth competition in experimental markets. Econometrica: Journal of the Econometric Society, 343-371."
This application provides a webpage instructing participants how to get paid. Examples are given for the lab and Amazon Mechanical Turk (AMT).