Some similarities to a prototype sold at prop store...
Prototype Stormtrooper Helmet
The mouth looks a bit different, just can't tell if its just paint...
The paint used for the decals are different, the one from prop store is grey where ebay version is black.
The black brow on the prop store looks textured or fabric, ebay looks to be smooth rubber.
The tape holding the grey bubble lenses seem close, same with the face half attaches to the back back half, both seem similar.
Not sure how many of these prototypes were made, you would think one or two until Lucas approved a design (then they would be ditched/tossed???). I have no clue, but without some sort of paperwork I wouldn't spend the $$$$...
Curious how prop store listed as a prototype (before production) but ebay listed the helmet as created after the filming.... No clue the real history, I tried to find the section on starwarshelmets.com but could not find the specific quote mentioned in the ebay link. Found it, halfway down the page...
http://www.starwarshelmets.com/Original-stormtrooper-armor-helmets.htm Made after production...