There is a collector's market but you have to spend a lot of time educating yourself about them, because there are a lot of really, really good reproductions out there that look original, but aren't. Plenty are "aged" as well and have "old" dates etched into them, nice designs and scrimshaw artwork - but aren't original. The guys and girls who made them weren't necessarily trying to deceive anyone, but simply building them for themselves and to their own taste. But the result is, it's a hard article to evaluate without holding it in your hand.
I personally wouldn't buy one without seeing it in person, just because even a seller can believe that what they have is original when it might not be. Some folks who just want a nice powder horn and don't care about authenticity, might. So the value online vs. "in-person" could be markedly different.