When I sell someone a knife, I provide them with what I call a "proof of life" photo. I take a picture of the knife in front of my computer screen with the most recent communication from the buyer clearly visible on the screen. That way they know that I do in fact have the item that they are buying.
Of course, if someone provides you with a "proof of life" photo it doesn't guarantee that they will actually send you the item, but at least you know that they do in fact possess it. Some scammers may use old photos of items they no longer have, or they might use photos taken from the internet.
If you have any concerns about a persons honesty, you might request such a photo. But then again, if you have such doubts, maybe you shouldn't deal with them at all.
Like this-