In a truly free market, the same good can't sell for two different prices

If gold cost more in London than in New York, traders could buy it cheap in one city and sell it dear in the other until the gap disappeared — the law of one price says arbitrage should erase any price difference for an identical good, once transport and information costs are accounted for. Real markets bend this constantly, through tariffs, shipping costs and simple ignorance, which is exactly why the gaps that briefly persist are so profitable to close.

— Wikipedia contributors, Law of One Price — Classical arbitrage principle in economics