Law of one price

"In an efficient market all identical goods must have only one price."

1. financial markets are perfectly liquid, transparent, and free, with no controls, taxes, or transaction costs, etc;
2. goods markets are perfect, with international shipment of goods able to take place freely, instantaneously and without cost;
3. there is a single consumption good common to everyone; or
4. the same commodities appear in the same proportions in each country’s consumption basket

thanks to: wikipedia.com