Vet the person--talk to them and see if they sound legit. I know this sounds abstract but the best way to detect a scammer is to ask them lots of questions, including personal questions. Non-scamming buyers/selling never take issue with this, I've even picked up the phone and chatted with people before doing a transaction. If the buyer or seller is irritable, impatient, rude, suspicious, flakey, theyre prob a scammer.
Also, stick with PayPal G&S or some escrow service. Legit buyers/sellers never expect to use PayPal Friend & Family.