Consider taxes and shipping costs. There may be taxes or duties on your purchase, especially if the transaction is international. Factor in shipping and handling charges as well to determine the total costs. You may also have to pay for shipping if you want to return the item. Ask about insurance.
Some auction sites provide insurance to buyers.
Be sure you know how insurance programs work and how much they cover. Pay the safest way. Credit cards are generally the best way to pay because you have legal rights to dispute the charges if the product or service is misrepresented or never delivered. You also have limited liability if your card number is stolen or misused. You don't necessarily have the same protection when you are billed through other third parties such as your telephone company.