You guys aren't getting it. Please allow a critique of the test.
The seller has offered and priced a stamp they represent to be a 4L1 as they have described it. They have offered no warrant or proof that it is the valuable item as listed in the catalog. They leave it to the viewer to accept or reject their description as given. Period.
You have offered a comparison between a similar item as sold by Siegel, and you have asserted that the two should be identical. However, your assertion, based on a comparison of pictures and not the actual specimens, is subject to expert verification which you have not offered. Moreover, and this is a critical point, because the PF and other agencies do not guarantee their opinions, if one were offered in court its claims would be subject to legal verification and testing. Unless you can show that there is no possibility the stamp in question could be an uncataloged variant of what you are using as your benchmark for authenticity, it's your opinion against theirs that the stamp offered is not authentic. You have
asserted your case but have not legally
proved it. Therefore, a disinterested third party,
ebay, is constrained to allow the seller to state his belief, without guarantee, in the offering of his item for sale. Since the seller has offered no guarantee, it is up to the buyer to decide for himself by whatever criteria he deems appropriate. [Here let me add my opinion that feedback ratings and customer testimony are no substitute for real evidence.]
I am of the opinion that no responsible philatelist should attempt to purchase an item of such value without some sort of guarantee of authenticity, so that they may have recourse for a return and refund if more adequate testing proves the item to be derelict. Moreover, the fact that Siegel, and many reputable auction houses, allows items in their sales to be placed on extension so such testing may take place, even when evidence of previous certification is offered at sale, stands as evidence that guarantees of authenticity are not to be assumed in this trade. Any buyer who pays this much money for a stamp offered in this manner under the conditions given, who cannot state the reasons for his conviction, is ill advised to make said purchase. But the venue,
ebay, is under no constraint to curtail the offer against the preferences of the seller. The "court of public opinion," has no legal standing to require such a thing.
Now, anyone who really wants to force the issue can take it to court in a lawsuit, but it will be on your nickel, and everything I have said here will be brought to bear. But be warned, if you persist in making the claim that the seller is doing something illegal, you could be sued. Some of yopu have loudly said the seller needs to be driven out of business. That statement is actionable, and so would any moves on your part to try to implement it. If this seller ever began to think that your efforts to drive him from his livelihood were actually a serious threat, you could easily find yourself in costly litigation.
The alternative to ALL of that is what I said earlier:
NO RESPONSIBLE PHILATELIST SHOULD ATTEMPT TO PURCHASE AN ITEM OF SUCH VALUE WITHOUT SOME SORT OF GUARANTEE OF AUTHENTICITY
If you really want to make the hobby a safer place, then militate for an environment in which authenticity guarantees are good for the life of the issuer/seller.