I say, and with God's success: The trader is not required to inform the buyer of the amount of his profit, whether little or much. However, if his action involves deception and trickery towards the buyer by selling at a price higher than the market price while misleading him otherwise, then his sale is disliked due to the sin involved. If there is no deception towards the buyer but his profit is excessive, then there is a type of dislike in it, and God knows best.