What is the ruling on selling an item requested by an individual at a price higher than its purchase price, knowing that this is its true market price, and what is the ruling on stipulating a division of profit when selling an item requested by an individual?

As long as you are an agent for your friend for purchasing, and you have bought on his behalf, then you are not allowed to increase the price at which you bought. You must return any excess amount you took unlawfully. It is permissible to stipulate a known amount or a known percentage for selling on his behalf.