I once read a novel where mc transmigrates to a xianxia or magical world. Acquires a system that allows him to buy and sell resources through the universe. He buys sells resources that are common on his planet, and buys rare resources and techniques from other civilisations. His power scales quickly and he leaves the barren continent to one with rich energy. I remember there is an array that covers the whole continent that prevents any violence. He starts a shop in the city there to hide the source of his wealth. He becomes the most powerful person in the world. We find out later that the former most powerful has been fighting off an encroaching invasion from another world for centuries.