I live near Boston, where there have been changed recently in how supermarket produce is sold. Heads of things like lettuce and broccoli used to be sold by the unit, but are now sold by the pound, as is most produce. At Trader Joe's, onions, bananas, and other things supermarkets sell by weight are sold by unit. I am still working on a 7" diameter sweet onion bought at Christmas for 79 cents, the unit price. (The clerk and several shoppers marvelled at it.)
I can understand unit pricing at farmers' markets and stands, where the vendors don't necessarily have scales with them, but it's odd to me that pricing produce by weight is not universal practice in other venues.