Is it permissible to buy a mobile phone from a friend at a reduced price and sell it in the market for a higher price without informing him?

There is no blame upon you for that, especially since the seller is the one who determined the price. Buying and selling are contracts of negotiation, and what is customary in them is the eagerness of both the seller and the buyer to gain profit. So, if mutual consent is achieved between the two parties, the sale is valid, because the Messenger of Allah, may Allah bless him and grant him peace, said: "Indeed, a sale is by mutual consent."