From a 1956 issue of the then-newly rebranded Friends Journal, an explanation of the ethics behind providing a fixed price for goods:
Whether the early Quakers were consciously trying to start a social movement or not is a moot point. Most likely they were not. They were merely seeking to give consistent expression to their belief in the equality of all men as spiritual sons of God. The Quaker custom of marking a fixed price on merchandise so that all men would pay the same price is another case in point. Most probably Friends did this simply because they wanted to be fair to all who frequented their shops and give the sharp bargainer no advantage at the expense of his less skilled brother. It is unlikely that many Quakers adopted fixed prices in the hope of forcing their system on a business world interested only in profit. That part was just coincidence, the coincidence being that Friends hit upon it because of their convictions; the system itself was a natural success.
— Bruce L Pearson, Feb 4 1956
Ironically, this practice probably accelerated inequality, since a wealthier person will be willing to spend more on a product than a poor person, so the haggling would act as a counterbalance to wealth concentration.