Is it permissible to sell a product on one website at a higher price than its original price on another website, by purchasing it from the original website and shipping it directly to the buyer after confirming their receipt, knowing that the selling process includes an accurate description of the product, the possibility of return, shipment tracking, and seller evaluation?

This method of selling a described item for future delivery is a sales contract between the questioner and the buyer. It is only valid through a Salam contract, and one of the conditions for the validity of a Salam contract is the full payment of the price at the time of the contract. This condition is not met here because the intermediary website receives the payment and does not transfer it to the seller until the buyer receives the goods within 21 days.