What is the ruling on working as a dropshipping intermediary between a seller and a buyer from eBay, where a seller's item (for example, priced at $50) is re-listed at a higher price (for example, $55) on the same website or Amazon, so that the intermediary receives the difference ($5) after the buyer receives the item and without returning it, even if it is defective?

Mediation between a seller and a buyer and taking a commission for it is permissible, provided the party from whom the commission is taken is aware of it. As for your statement: "without returning it if it has defects; because if it were so, the item would be returned to the seller, and the buyer would be compensated, and that would be within approximately 20 days"; if what is meant by it is the seller's guarantee against defects, then there is no harm in it. However, the question remains about the ruling concerning old defects discovered after that period if the seller stipulated immunity from them.