If you look closely at the perfs, you will notice this stamp is different compared to the others previously sold. The last seller also sold one that had been previously sold, but they are clearly different then the current one for sale.
Also, it doesn't seem to be a lucrative "cartel" since the sale price has been consistently going down.
For a grand+ I am surprised there is no cert.
It is amazing this stamp comes up on
ebay so frequently given there are less than 1535 of them issued.