#1, Photos of the actual item does prove something, It shows the actual item.
Wrong. As I already explained, you have no idea if it is the actual item for sale or a copy of an image the seller has downloaded from somewhere.
There are many new photographers AND new forum members who may not know what every camera or camera accessory looks like. I don't. I enjoy looking at other members products.
I was looking to get into m4/3 recently and had NO idea where to start, Fuji this, Nex that, Oly 5, Rx-1. I had no clue what all those looked like. Fortunately members selling their M4/3 products showcased them well, very well, so it made my decision making easy.
As a buyer, you should be researching your item before buying it here or anywhere else. Googling those cameras will bring up photos and reviews, and will give you a lot more info than looking at a seller's photos of the same item.
#2, It's a courtesy to other members. It's just called being nice and helpful. That's one of the reasons that ALL of my camera gear I have bought from members of this forum. And will continue to do so.
And if the camera the seller is selling is his only camera, it can be a hassle for him to photograph it. And when he does, the photos look just like the camera listed at B&H. Surprise, surprise.
As a seller, you're not doing a buyer a favor. They, as a buyer, are doing the favors by buying your FS items.
IYO. A FM seller giving me a good deal is doing me a favor, IMO.
I generally post images of the items I sell, even though I don't require a seller to do likewise when it serves no purpose for me.