Hi Tomahawk,
Welcome to the Forum!
The seller's right: it's definitely in the style of Northwest Coast Indians, and they did indeed make copper daggers.
Now, for the nasty question: is it a "genuine Indian Dagger?"
Dunno.
Here is a pdf containing images of "genuine" PNW metal daggers, and this one doesn't look the same, either in styling, blade shape, or patina on the blade. If yours is genuine, it's missing a good chunk of its handle.
Trouble is, it could have been made relatively recently by a Tlingit artist, which would, indeed, make it a genuine Native American copper dagger, although not an antique. If you're asking about whether it was made in the 19th Century or before, my guess is not, but I could be wrong.
It could also have been made by a white artist imitating the distinctive PNW style. Without further provenance information, it's quite hard to tell.
F