mercenary (n.)

early 12c., "a meeting at a fixed time for buying and selling livestock and provisions, an occasion on which goods are publicly exposed for sale and buyers assemble to purchase," from Old North French market "marketplace, trade, commerce" (Old French marchiet, Modern French marché), from Latin mercatus "trading, buying and selling; trade; market" (source of Italian mercato, Spanish mercado, Dutch markt, German Markt), from past participle of mercari "to trade, deal in, buy," from merx (genitive mercis) "wares, merchandise." This is from an Italic root *merk-, possibly from Etruscan, referring to various aspects of economics.

The god Mercurius was probably the god of exchange. According to [Walde-Hoffmann], the god's name was borrowed from Etruscan; in principle, the same is possible for the stem *merk- altogether. [de Vaan]

Meaning "public building or space where markets are held" is attested from late 13c. Meaning "a city, country or region considered as a place where things are bought or sold" is from 1610s. Sense of "sale as controlled by supply and demand" is from 1680s. Market-garden "plot of land on which vegetables are grown for market" is by 1789. Market-basket "large basket used to carry marketing" is by 1798. Market price "price a commodity will bring when sold in open market" is from mid-15c.; market value "value established or shown by sales" (1690s) is first attested in the writings of John Locke. Market economy is from 1948; market research is from 1921.