What is the ruling on a reverse auction in which the seller offers an item at a specified price, and buyers submit bids at lower prices, and the winner is the one who submits the lowest bid, provided that he is the only one who submitted that bid? And if it is impermissible, what should I do as a software engineer who has agreed to develop a website for this transaction?

There is no objection to the seller selling his goods for less than their value. However, the described characteristic is unclear and indicates ambiguity, as the buyer's abandonment of competition for the lowest price leads the matter to the lowest conceivable value. This would not be done by a sane person except for the purpose of deceiving buyers and making them believe that the sale is at the lowest stated price, when in reality it is not. This is considered a sham tender, which is forbidden due to the deception and fraud involved. And due to the saying of the Prophet, peace and blessings be upon him: "Whoever deceives is not of us." It is not permissible to work or cooperate with one who does so, because of Allah's saying: "And do not cooperate in sin and aggression" (Al-Ma'idah: 2).