Ken,
All of the 'Great Britain' sellers are thought to be the same person. This 'British cartel' person has previously had listings where a cert was known for one of the stamps but it was not being mentioned since it would nullify the substantial alteration that the stamp had received.
But I think your point is interesting. Statistically it seems difficult to deal in a large number of higher dollar stamps over time and not have some percentage that came with certs. And then one most ask one self, why would a dealer not go and get certs for many of these, knowing that they would seller quicker and at a higher price if there was a cert attached?
I admit that I do not fully understand why you are filtering with stamps 'that have been sold' as opposed to all listings. I have seen a handful of listings from the British seller which do indeed have a cert although I do not recall if they sold. But what is obvious and can be stated as fact is that either these sellers are all the same person or the material is moving between them in some kind of relationship. This has been proven without a doubt. Also proven are some listings from this seller which were originally sold in hinged state in Spiegel auctions but later popped up on
ebay as 'mint, never hinged'.
It would be interesting to see if all of the 'sold' items went to buyers in the same country. If, for example, there are a lot of buyers who are willing to plunk down that kind of money for a cert-less stamp, you would think that they would be looking for possible recourse paths if the transactions go bad. Having the seller in another country certainly is NOT helpful if legal recourse is called for.
And lastly, it would also e interesting to see how many of these dealers are recognized as being members of a major philatelic organization(s). Again, you would think that buyers would be more likely to support those sellers which were members of APS or ASDA or some other organization when trading in higher dollar material.
Don