Is it permissible to inform the buyer that the square meter price for the land has been bid at 285 riyals, or must one say that it has not been bid on before? And is the one who says so considered sinful?

If the buyer understood that the bidding was after the prices had dropped, then this is a form of deception and lying. The Prophet, peace and blessings be upon him, forbade deception and mentioned that truthfulness and clarity bring blessings, while concealment and lying remove them.