I take it to be completely authentic. A hoaxer who built a replica would say "I acquired it from a collector who wishes to remain anonymous." This is meant to imply that a studio prop man stole it and sold it to the current owner. A lot of communicator fakes were shopped with that story.
But this guy selling the phaser rifle is right upfront about his own name and that he himself built it and lent it to the show. And he has papers and vintage photos of himself as a younger man demonstrating the prop.