Isekai⇔Chikyuukan de Kojin Boueki shitemita

One day, Wataru stumbles upon a gate that allows travel back and forth between Japan and another world. Beyond the gate is a fantasy-like world. Seeing a town whose civilization was undeveloped in many respects, he realizes the potential for great profit. In other words, bringing sugar, spices, coffee, hygiene products, and the like to the other world and making a fortune. Striking it rich in the other world, Wataru suddenly realizes something when he sees a potion for sale: "Maybe this would be better in the other world?" Wataru makes a killing bringing potions that instantly heal the user's wounds to earth. He also buys beautiful slaves, and before long, he is doing great on earth too. (Source: Tokuma Shoten, translated)