That sucks. Really sorry to hear that Matthew. But personally, I'd probably never consider buying any supposedly authentic weapons from a guy that was admittedly selling repros at the same time. Just too risky.
When he told you they were authentic to the best of his knowledge, had you already made him aware of the fact that you knew he was also selling repros? If not, that might've been the difference between him thinking he could try to pull a fast one on you, or thinking that I should just be honest with this buyer since he already knows that I sell repros...
I'd just contact the seller, and tell him that you received the weapons, and they are most definitely repros - no questions about it. And for that reason, you'd like to return them to him for a full refund (including the return shipping cost). If he plays hardball after you're initial request, I'd even drop a mild "threat" in there that you're going to rat him out to eBay for trying to pass off repros as originals. Ebay frowns greatly on that. Mention you'll hit him with negative feedback as well...
So just contact the guy, be polite and honest, and tell him you're not going to make a big stink about it, but that you just want your money back. Hopefully that does the trick. Good luck, and let us know how it goes...
Dale (DSJ) might also be able to chime in with some good advice, since he absolutely loves busting eBay scam artist like this!
