What is a mandi licence and do ordinary farmers need one to sell?

A mandi or APMC licence is required for traders, commission agents and other functionaries who buy and trade in the market, not for a farmer selling his own produce. As a farmer bringing your own crop to sell, you do not need to buy a trader licence; you just need to be registered as a farmer or seller at the mandi, and for eNAM you need an eNAM farmer registration. The licences you hear about are the trader licence, commission agent licence, and in many states a unified or single licence that lets a trader operate across multiple mandis. So the answer is that the buyers need licences, not the selling farmer. Confirm the registration formality for sellers at your specific APMC, as a one time seller registration is sometimes asked for.