Is it permissible to sell a watch that has been repaired after a button fell off for the same price that was set for it before the repair, with the seller knowing about the repair but the buyer not knowing?

It is obligatory for you to disclose the repair of the watch to the buyer, if the repair diminishes its value or reduces their desire for it. As for profit, it is permissible to sell the watch at any price you wish, as long as there is no deception or misleading of a buyer who is ignorant of the market.