This is quite natural. Sellers must be concerned that the buyer is legitimate and the payment for goods or services rendered is drawn on a valid account. Even so, in a very short time, millions of individuals have come together on the web to buy and sell all sorts of items, large and small, cheap and expensive. In e-commerce, reputation itself becomes a valuable commodity. Read about how eBay fosters trust at its Safe Harbor site.
Reputation is formalized and amplified by trusted third-parties.
These are not in themselves perfect solutions.