What is the ruling on buying a used device from a distant country and selling it at a higher price in the country of residence to benefit from the price difference and the difficulty of shipping, and is it obligatory to inform the buyer of this, and what is the ruling on selling the device by auction if its value exceeds the purchase price?

If the sale has been made and the goods have been acquired, then it is permissible to sell them at any price. There is no obligation to inform the buyer of the purchase price or the amount of profit, except in the case of trust sales (buyu' al-amanat). A condition for the validity of the sale is the seller's satisfaction with the price. It does not matter whether the profit is small or large.