What is the ruling on selling books at a price higher than that stipulated on their covers, while effacing the original price and placing a sticker with a higher price?

It is permissible for you to efface the written price and replace it with a higher one if the book is your property and the buyer consents, for there is no specific limit to profit in trade; rather, it is subject to the mutual consent of both parties, as affirmed by His Almighty's saying: "Except for a trade by mutual consent among you." The Sunnah (definition) also contains evidence permitting a large percentage of profit, such as 'Urwah's purchase of two sheep for a dinar and selling one of them for a dinar. However, if you are an agent, you are not permitted to change the specified price without the owner's permission.