What is the ruling on purchasing school supplies from a wholesaler, with payment made after the exhibition ends, and returning the remaining supplies?

This transaction might mean that you are agents for the owners of the goods, which is permissible. It might also mean that you are buying the goods with the condition that whatever is not sold is returned, which is known as a "sale for disposal" (بيع التصريف). This type of sale is a matter of scholarly disagreement.

Ibn Uthaymeen said regarding the "sale for disposal": "I sell you these goods. Whatever is disposed of (sold) from them, let it be sold, and whatever is not disposed of, return it to me. This transaction is forbidden because it inevitably leads to uncertainty (ignorance), as neither the seller nor the buyer knows how much of these goods will be disposed of... It has been authentically reported from the Messenger of Allah, may Allah's prayers and peace be upon him, that he forbade transactions involving gharar (excessive uncertainty), and there is no doubt that this is a form of gharar."

He clarified another permissible method: "The owner of the goods should say: 'Take these, and sell them by agency,' and he should set a fee for his agency... In this case, the second person would be an agent for the first, for a fee."

In another instance, Ibn Uthaymeen favored the permissibility of the "sale for disposal," saying: "Or, according to the preponderant opinion, he might say: 'These – for example – ten boxes are yours for one hundred, each box for ten. Whatever is not disposed of will be returned for its proportionate share of the price.' We consider this permissible because neither party incurs harm, and there is no injustice in it. The owner of the goods is ready to accept what remains. Therefore, the important thing is to specify the price of each item, and then it will be sound."