This is typical of the counterfeit guns sold on all of the auction sites. It was intentionally "aged" to fool buyers and the vague description's wording tells me the seller knows it is a fake. He intentionally did not claim the gun was an original but hints strongly that it is.
A word to the wise is, never buy a gun that is an "antique" without a guarantee that you can return it and get your money back if your not satisified. After getting the gun, take it to someone knowledgeable about the real gun and have them inspect it.
In this guns case, the fact that all of the screws will turn out to be metric threaded will prove the gun is a fake.
I agree with the post above. The real Remington New Model Army barrel is stamped,
PATENTED SEPT 14, 1858
REMINGTON & SONS, ILION, NEW YORK U.S.A.
NEW MODEL