What would you think would be a fair price? Trying to figure out what I am going to say to him tomorrow. He says he Guarantees its authenticity. But I have to ask, What are you guaranteeing?
I'd price it as a mismatched bolt rifle with a replaced stock, a "restoration". BOTW has stuff like that in it, a good bit more than is reasonable. It's priced and pitched to appeal to an advanced collector, but an advanced collector is going to (at best) be ambivalent about it being in BOTW, and may look at it as a negative, in addition to noting all that you've read here on it.
So, take an original rifle and cut it in half at least twice.