FLORENCE, Mass. — John Spineti started selling plump tomatoes and shiny squash at farmers’ markets in the early 1970s and saw his profits boom as markets became more popular. But just as farmers’ markets have become mainstream, Mr. Spineti said business has gone bust.

Farmers in pockets of the country say the number of farmers’ markets has outstripped demand, a consequence of a clamor for markets that are closer to customers and communities that want multiple markets.

Some farmers say small new markets have lured away loyal customers and cut into profits. Other farmers say they must add markets to their weekly rotation to earn the same money they did a few years ago, reducing their time in the field and adding employee hours.

“It’s a small pie — it’s too hard to cut it,” said Mr. Spineti, who owns Twin Oak Farms in nearby Agawam. Mr. Spineti, who was selling vegetables and small fig trees, his farm’s specialty, at the Wednesday market here, said his profits were down by a third to a half over the last few years.