Also common everywhere I've ever lived in the US: central New York, rural Massachusetts, Iowa, etc. Usually, there's an asking price, and you leave your money in the box or envelope. That's typically if someone specializes in a particular thing, and has it often and has a price for it (corn, eggs, honey, veggies, etc.) Sometimes if it's someone with a more random assortment of stuff, you leave what you think it's worth.
How do people with farm stands collect the money if they don't do this? Do they have to leave someone sitting by the side of the road all day?