I've been trading online for 10 years. I've owned several mailorder companies and I know how you can protect yourself 100% from a fraudulent seller.

Never wire transfer money to a seller you have not had positive experience with at least one time. Never! More importantly, the seller should have a few hundred positive feedbacks before you consider sending out money via check or transfer.  You have no recourse whatsoever if you do not use a credit card or Paypal to make online purhcases. Most credible resellers will have a credit card processing account or a Paypal account. I am always cautious with buyers that do not have Paypal or have a very high feedback score. If you are buying from a private person and not a business use Paypal - do not send checks or moneyorders. Remember, make sure the seller has a good feedback score.