Portal of Fortune: The Multiverse Merchant

Chris Leary took over a corner shop that was losing money. He took loans for his girlfriend's treatment, fell behind on the repayments, and the lender sent men to the shop. While they were there he found that the back door of the shop opened into another world. The first crossing put him in a wasteland after some collapse. He located a base of survivors, sold them food and was paid in gold, silver and jewellery, and one run covered the debt. The base then put in for more than food, medicine included, and while working on that he crossed into the realm of the immortals. There he made an agreement with the Dark Lady, cooked food from his own world against her pills, which covered the medicine. He also learned that the wealth coming out of the wasteland was tied to whether the people there stayed alive. He carried the pills back and the base came through, and the two sides settled into a standing arrangement. After that he ran goods between the worlds through the back door of the shop.