Personally, I'm going to agree with the seller, and doubt this rifle was ever a flintlock.
I've studied Hawks work some, and I've only seen one other swivel that I think was originally made as a percussion, and that's the one I own.
I base this on the small oval, engraved on the forward lock plate. If the gun was originally a flintlock, the engraved oval would be under the frizzen spring, and there wouldn't be much point in engraving something that would be under the spring.
Plus, the 'without a doubt' flint guns that I've seen, or seen pictures of, don't have this oval engraved there.
The forward portion of the lock on my swivel gun is exactly the same as this guns, with the engraved oval, the bit of engraving around the screw, and what looks like the remnants of a pan around/under the percussion drum. I don't actually think this is the remnant of a flintlock pan as used on the gun, but more just a remnant of the thought of one, and is now just a small projection to help keep percussion flash from taking its toll on the lock plate.
I agree that at some point it wouldn't be surprising to see this gun turn up as a flint though!
As for the engraving, Hawk was a very fine engraver! His hand was light and fine, and at one point he evidently spent some time in jail for forging either paper money, or the plates to print it.
Before I would buy it, I would suggest looking closely at the silver inserts in the PB and inlays behind the cheek piece, and some of the engraving that's on the lock on this gun, and decide for yourself whether it's Nicolas Hawks work or not. The decision might or might not make a difference in how you'd value the gun.
Also, it's interesting to note that when the barrels were cut back about 6" from the breech end, that the brass side pieces were cut in two ahead of the locks, the extra length removed, then the two pieces riveted back together again at the new length.
This is a nice gun, and it's nice to see another unpublished one by Hawk (at least I haven't seen it before), and at an appreciating price!
John