While the sight is shocking, the deal itself is even more disturbing.
Now, to be fair, I did something almost as scary two years ago. Truck gun is an SKS. Hunting season (yes I do hunt with it) was extremely busy at work then. I barely got any time off at all and what I did get was rushed. I had used that rifle to take a doe at 50yds then left it behind the seat for for the winter. I TOTALLY forgot about it. When I started shooting gophers, I went out the first morning to put the .22 behind the truck seat. And there it was!!! My absolute horror! My father would kill me if I hadn't already. Needless to say, no gopher shooting occurred that day. Even at that, the rifle was not as badly rusted as that since I do use oil on it. It was a little rusted at the muzzle (I had to touch up the crown a little) and the piston was rusted. NOT seized, mind you, but well rusted. It cleaned right up in an afternoon and shoots the same.
Never want to see that from me again. Even my wife jacked me up, and rightly so.
That being said, I would not have sold it for market rate like that. What the hell is wrong with people??
I did have a friend, a very good friend, who used his SKS (along with other rifles) during an outing with Boy Scouts. We both did. The boys fired thousands of rounds of .22LR and hundreds of x39, along with a few boxes of .308 and .30-06. It was a very snowy, wet day and when he went home at the end of the day, he put the rifles, all in cases, in his basement for cleaning, but got caught up in the kids and wife wanting stuff and he forgot them. His wife put them away the next day. He didn't hunt the following year because he was in school and brought me these rifles when he finally opened them to get ready to hunt last year. They looked a lot like the one you have here OP. The piston was seized in the gas tube, EVERYTHING was furry and stuck to the material of the case (it tore pieces off of the case). I soaked it lots, stripped it, and got the pieces apart and freed up. I soaked the gas tube in Ed's Red for a week. It came free with some persuasion. I cleaned it up, touched up the bluing, and it still shoots as a plinker should.