I have a few Italian helmets with stencils. I believe them all to be original.
My best advice is to get a few good books. Paolo Marzetti's "Combat Helmets of the World" (fourth edition to be released this month) and "L'elmetto Italiano", Bossi - Nogueira are my two favorites.
I have been told that stencils should match known originals exactly, with no variations, as most redone helmets are incorrect in some way. This is only true for pre-RSI period helmets. Many RSI helmets were painted by whomever the best artist in the unit was. This makes these helmets highly suspect. The RSI helmets in my collection were bought from a trusted source but I still wonder at times.
As always, if you have been collecting for a while, "gut reaction" goes a long way in deciding what you are comfortable with.
The helmet on ebay may be real. I would ask for better pictures.