Shaykh al-Islam Ibn Taymiyyah (rahimahullah) said:

“It is not permissible for the traders to sell to the bargainer at a certain price and then sell to the one who does not bargain (i.e., the easygoing buyer who does not haggle) or to someone who does not know the actual price for more than that price. This is something that sellers are criticized for.” (Majmu‘ al-Fatawa, 28/75)

And he (rahimahullah) said:

“If the buyer is easygoing—and does not know the actual price of what is being sold—it is not permissible for the seller to deceive him by overpricing that goes beyond what is customary. Rather, the seller must sell to him at the usual price or close to it. If the seller deceives him by overpricing excessively, then the buyer has the option to either annul the sale or let it stand.” (Majmu‘ al-Fatawa, 29/359)