The point is you offer someone who registered for mail in to simply sell you their ballot along with a signed envelope. I would vager doing this for a few votes would be fairly easy. Certainly plenty of poor voters would easily sell a vote. Idk about 10, but for 20-50 bucks? For sure. I would imagine someone selling a vote would be less likely to come forward in some respect too, so if you target the right people, at some minor scale you would even get away with it no problem.
Yes this is obviously not happening, not at scale anyway, and certainly not enough to flip the election. But the idea that this is particularly hard in itself, let alone impossible, is absurd.
At least humans have the highest sensitivity specifically around 555 nm (green).