A solid international purchase contract names both parties clearly, describes the goods precisely (specification, quantity, quality standard), states the price and currency, sets a delivery date and location, defines how quality will be inspected and accepted, sets payment terms, states what happens if goods are late, defective or short, and says how disputes will be resolved and under which country's law.
For smaller or repeat orders, a full formal contract is not always practical, but the same essential terms should still appear in writing, even if only in a confirmed pro forma invoice and purchase order exchange; verbal agreements are very difficult to enforce internationally.