Is it permissible to profit from family and friends by buying electronic devices at a reduced price and selling them to them at market price without informing them of the reduced price?

If you are an agent for them in purchasing devices, it is not permissible for you to take the price difference except with their permission, for an agent is entrusted, and the excess belongs to the principal; based on the Hadith of Urwah ibn al-Ja'd. However, if they are buying from you as a merchant, then you are allowed to profit from them.