What is the ruling on a friend taking a 5% commission on the sale of an item owned by the questioner, knowing that the commission increases with the price, and that he is the one who sells the item to the buyer first?

This transaction falls under the category of brokerage, which is permissible as long as it does not involve aiding in something forbidden. It is considered a type of ju'alah (reward/stipend). A condition for it is that the fee must be known. It is permissible to make it a percentage of the profit, according to one narration from Imam Ahmad and Ibn Sirin. So, if your friend delivers the merchandise, he gets 5% of its price. As for his statement, "when a buyer is found, I will sell it to him and then he will sell it to the buyer," this is a permissible buy-and-sell transaction if he takes possession of it and then sells it to the other party.