“I bought a pair of Rockport dress shoes at standard price in Marshalls for $35.99. The next day I happened to be in a frum shoe store and the exact same pair was $115. “
Over- or under-charging by more than 1/6 is asur.
To the best of my knowledge, Marshall’s sells closeouts.
In addition, to establish that something is asur because of ona’ah, you have to take into account a number of factors, including:
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prevalent selling price (not just compared to one or two stores)
location
level of service
Chances are, a claim of ona’ah in this case would not stand up in B.D..