There is no 100% surefire way to not get scammed, however I have tried to add pointers in the sticky to help. I also added this last section at the request of a POTN member, just to provide at least some advice to others not exactly sure how to wheel and deal on the forums. Also one of the other bullets I have in a different section calls out googling the paypal address of the person you are about to pay. If you are in the first group to get scammed, this won't help, but hopefully in the early phases of the scam, the forums are all abuzz with the email addy and you can save your funds.
It is also probably very true, that if you buy and sell online regularly, at some point somewhere, you will end up falling victim. My sticky hopefully stacks the odds in your favor though.
1) Remember that you hold all personal responsibility for how you send payment to the recipient. Nobody else outside that transaction is to blame if you lose your money on a bad deal.
2) There are many forms of payment, but the most popular to pop up is Paypal personal payments (otherwise known as Gift, money owed, etc). Think of this as no different than sending straight unadulterated cash to a recipient via uninsured, unregistered first class mail. Would you do this if you trusted the person at the other end? Sure. Would you do this if you don't know the other person but really want in on a good deal? Your call, but also your responsibility.
3) Look at all the alternatives when you make a payment: Paypal, Paypal Personal/Gift, Paypal Masspay, Money Orders, Checks, Google Checkout, Chase Quickpay, and many other options.
4) Many will suggest to make sure your payment through paypal is done via a credit card in order to garner other protections. A good idea, but be sure you know your credit card account terms, and the Paypal terms.
5) If you send money via Paypal personal gift payment, Paypal will almost never help you, and you certainly cannot pin blame on the forums or others outside the transaction for having sent that to an unknown party. Always double-check the seller's reputation first and foremost, then decide whether you would send cash to the person with no traceability for the item, and if the answer is yes, then using Paypal personal payments is really no different.
6) If the seller and buyer both don't want to budge on the paypal gift idea, perhaps the two could compromise and split the fees down the middle. I personally have used paypal personal payments with no issues at all, but I have also split fees with buyers in the past, as well with sellers. This leaves the normal Paypal protections in place, for what they are worth.
Motto to Live By
GREED or LUST for new equipment or a spectacular deal makes people do crazy things! Take some time and think about what you are about to do especially if it is a high-dollar item! Also ask yourself whether you could continue to make ends meet, if for some reason that money were to disappear and you received nothing. In the end remember that you, and you alone, were the one to send the money out the door, and that is a decision you ultimately have to live by.