Is the sale considered void if the agent does not inform customers that he is not the owner of the goods, knowing that the agent profits from raising the price of the goods and from fees paid by the seller?

There is no harm in the aforementioned transaction as long as the agent is authorized by you to sell the goods, and he does not sell them until he has verified their presence in the warehouse, and he sells them according to their description.

The agent stands in the place of the principal in the contract, and he is not obliged to inform the buyer that he is an agent. His selling of the goods is not considered selling what one does not own, because he is a representative of the owner of the goods.

As for your agreement with him on the price increase and his taking it for himself along with the known fee, it appears that this combines his fee being what you give him and what he takes from the amount exceeding the price. We have clarified that among the conditions for leasing is that the fee must be known, either as a fixed amount or a percentage of the profit, or a combination of both, as narrated in a tradition from Ahmad.