I have a couple of Mr. Maxwell’s screens. I bought them new, so I know how expensive they are. I would add one caveat to your comments and that is, when you buy them new, included in the price you get the benefit of his expertise in choosing which of his screens will be the best compromise for your intended use. Matched to the focal length being used, his screens can be a vast improvement. However, if you have a mismatch using a screen that is more suitable for lenses in a different range of focal lengths, you will not get the benefit his screens are capable of giving and you may find that you have gone from not so good to unacceptable.
This might be, perhaps, why he does not sell them without taking the time to consult with the buyer as to what screen will be the best compromise for the range of lenses being used. Also, the screens are very delicate and easily scratched. They are well packed for shipment, but after that, he has no control over their treatment. For that reason, I think he wants to keep things where he has complete control over any issues and does not have the problem of people who chose poorly wanting to return screens for refunds or exchange.