Is it permissible to buy goods for the mosque at a reduced price and sell them at a higher price for the benefit of the mosque and its facilities?

What you are doing, by telling vendors that the items you are purchasing are for the mosque so they may lower the price for you, is considered a forbidden lie in Islamic law. It is feared that this may involve consuming people's wealth unjustly, because merchants reduce the price as an act of drawing closer to Allah. The obligation is to repent to Allah and seek forgiveness from the owners of the wealth. This can be avoided by informing the vendors that the trade is for the benefit of the mosque, or that the expenses for its facilities come from donated funds.