Fabrizio,
I don't want to question your opinion, but this is what I think:
1) The final price of an auction on ebay is not a verdict about the authenticity of a collectible. A genuine shallow fuller was sold for $ 91
http://www.quarterbore.net/forums/showthread.php?t=5555
2) The fact that the seller didn't even mention in the auction that this was a shallow fuller (because he/she probably didn't know anything about it), is something in favour of the authenticity. Just think about the rejected General Cutlery M9s: they were all corrected worded in the auctions (I bought myself one many years ago)
3) I think that you would need very specific machinery to modify the fuller the way you suggested: why take all the time and effort, and then sell the bayonet not even mentioning it was supposed to be something very rare?
Just my thoughts.
P.S. What is the "M9 BLUE"?