The real farmers that drive in to urban areas tend to do the majority of their business with restaurants, often the rather better ones that want high-quality or unique produce (this is according to farmers I've talked to who referred to their farmer's market sales as 'nickel-and-dime'.)

Some of them do a half-off sale for everything they've got at the last half-hour of the market, that's good to know. Some are just buying from the wholesalers, too, that's something to watch out for as others note - but some of those run lower margins than the local grocery stores, so it's cheaper to buy from them.