90a- The Rashbam says that one who buys from wholesalers and sells to individual customers is Rabbinically forbidden to mark it up by more than 1/6th. This applies only to staples. Then how do grocery stores today work, as they certainly buy staples from wholesalers and mark the items up by more than 1/6th?
Barry Epstein, Dallas, USA
The prohibition was for retailers for whom the only difference was that the retailer sold it piecemeal rather than all at once.
Today, in contrast, the retailer has a myriad of costs, delivery, overhead, taxes, interest, etc., and all these are factored into the price (see Shulchan Aruch, Choshen Mishpat 231:20). Today the only prohibition is to sell over the market price, if such can be determined.
(See also ksuv-040.qa2, ksuv-040.1c1)