One, try and see the items before you buy them. This will prevent false claims of items being sent. Sometimes, a phone call might be helpful too.
Two, always use a service like PayPal to transfer money to a seller. That way you can always report the buyer and get your money back.
Third, always agree on a return policy.
Fourth, try and get as much detail as you can about a seller before completing the transaction. Details like Name, Location, Place of work, Mutual friends etc.
Please use the tips above and protect yourself better.