Having just a bit of street smarts when dealing on the internet goes a LONG way. As the old saying goes : If it seems too good to be true, then it probably is.
Additionally, something I always request are actual, quality, pictures of the item being sold (particularly in situations where the seller doesn't have much feedback and/or if the item is of considerable value). If the item is in-hand (which it should be), the seller should be able to take as many photos as requested. Yeah, someone could get photos from the net, but typically you won't find more than 2-3 photos of an item taken in the same environment and same lighting conditions. A quick Google search, to be sure they aren't photos grabbed from the net, is good practice as well.