I'm not sure how it's done in other places these days but, in Maine, fresh vegetables are usually sold from farm stands. Generally, the stands are placed along the roadside at the end of the farmer's driveway, and are unattended. Prices are posted on the wall and there is a container in which to place the money. I asked one of the potato farmers who was there one day if they have much of a problem with people taking potatoes without paying for them, and he said that it does happen, that there has been times when people have taken everything in the stand but that, more often, people leave more than they owe for what they are buying, since there's no way to make change. They might leave $5 for a $3 bag of potatoes. He said that it more than evens out in his favor, particularly when you figure out the cost in time of having someone at the stand throughout the day, and that no one has ever stolen the money.