Mandi is a designated physical space, market, where any buyer & seller can transact; & government mandi officials will record & observe all transactions for data logging, & to keep the issues away. Nobody has monopoly, anybody can come & buy. Think of it as a kind of transaction between a craigslist buyer & seller at local police station.

If buyer & seller trust each other, like between my family & farmers in out village, we used to buy the produce directly from their home.