It occurred to me recently when contacted by a possible scammer, that a good way to protect yourself would be to do the following. If you are considering buying an item from the person, ask for photos of the item that includes either their DL or radio license in the picture(s). I would insist on them being a licensed ham in order to do any deal. Also, A phone number is mandatory. If this person is hesitant in producing these requests, then do not proceed any further. If the seller is willing, have him or her pack the item and produce a photo of the shipping label. This may sound excessive but times have changed, and so have scammer's techniques. Fortunately, with paypal there is a complete trail that protects the buyer as long as the seller is verified. I have never been ripped off but I still maintain vigilance because you never know.
Of course, the photos are your best friend. Almost always, the scammer will use a photo found on the internet of the item. This just happened to me and I had to laugh out loud. You would think the person would have been more creative. A well presented photo with really good lighting and backdrop in pristine condition for a low price just screams a scam in progress. Little things like all caps in the subject line will cause me to be suspicious. Syntax and grammar are also dead giveaway's.
Good luck, be careful and don't get screwed!