Since this happened on a sealed, 400ft roll and the seller has tested it before, I believe it will only affect a few of the cassettes he made
I don't think so.
A 400ft roll of film is essentially a thick pancake with a core/hole in the middle. It looks like this entire pancake was taken out of its packaging and exposed to the light. The packaging of a roll of cine film (Kodak) is inside a black plastic bag, and this bag is inside a metal/aluminum can, and the can is taped shut at the end of production/confectioning; the metal can sits inside a carboard box. Opening the thing up involves taking the can from the box, removing the tape, lifting the lid off the can and opening the plastic bag. It's easy. It's also easy to close everything up again, including re-applying the tape, in such a way that it'll be hard to see if the package has been opened.
Here's an illustration:
The metal cans currently used by Kodak are gold colored, but the shape is virtually the same. Not shown in this picture is the textile tape (looks a bit like bookbinder's tape) that goes around the seam. Also not shown is the cardboard box new cans come in. It also looks like the actual roll of film in this image is much less than 400ft, as there is only little margin between the film and the metal can on a normal/full roll.
The light on your film has penetrated the area around the sprocket holes, mostly. Given the fact that the fogging is similar on both edges, it seems the roll was exposed to normal room light or even daylight, and exposure was fairly through - i.e., they took their sweet time having a look at the roll. It's to be expected that the fogging pattern will be similar across the entire 400ft roll. The leader of the roll would have been entirely fogged, including the image area. This will be true for the outer few layers of film. It's likely that the fog will protrude a little less into the image area on some parts of the film, but I don't expect any of this film to be unaffected.
The film you have is basically FUBAR. Too bad! It's best to purchase cine film directly from reputable sellers. I get mine from frame24.co.uk; I think in the US, there are more choices.
Sadly, one part of the current trend of using cine film for still photography involves a cottage industry respooling the film into 35mm cassettes, where quality control is mostly lacking. Most of the time, there are no big problems, but your case illustrates that things can go horribly wrong, and when it does, sellers are apparently not always upfront about it.