It is pretty common in Vermont and it is not only about veggies (meat and poultry too). You can just write a check or pay by cash and take change(if you need) from the cash register which is open. It is not based on the honor system (pay as much as you want) but on decency and trust, you have to weight everything and calculate price for goods by yourself but there is no one who is watching you.
It seems to me that such a system makes people show their best. Surely there is a percentage of people who may not act very well, but I believe that there are more good people anyway.