Thanks Norton! I found it, and looked at the gun in question. In my humble opinion, it's not legitimate. The rollstamp markings are engraved, not rollstamped. Someone went through a bit of work to make it look correct, but I'm not buying this one, or this seller's description. He seems to be leaving himself an out.
I've never heard of a 1881 (or any Marlin lever action) with a barrel that long, but I would never rule this out. That's not my reason for thinking it's not legit. The real reason is the rollstamp on the barrel looks too uneven and shaky to be a factory rollstamp. It's a good engraved copy.