Read the info that the moderators have regarding purchases. I usually try to only purchase from someone who routinely advertises and posts on hear cause they have their reps to lose if I report them. If in doubt about someone's legitimacy, I will use the "purchase goods" option in Paypal and pay the 2.9%. This way I am able to open a claim against someone.
In purchasing/selling online for well over 10 years, I have only thoroughly been burned on Amazon once and probably 3 times on Ebay. I was able to get resolution on Ebay each time, but on Amazon I fell for a scam where I sent the goods to someone who had obtained my email when they asked me to send pictures to a particular address. They then they sent me a phony email which appeared to be from Amazon telling me to ship the item. I sent the goods and then a few days later tried to get the item back but I couldn't get the Post Office to even track and find it, let alone stop the shipment. What I learned is to never exchange emails unless I build a relationship or trust someone and never ship without verifying payment. Very expensive lesson on my part.