If they sell for $1000 new then the dealer bought it for $750-$800 (wholesale price), so to start with that's what it was "worth" brand new.
If a used gun was taken in trade by a dealer they certainly wouldn't pay more then the cost of a "brand new one", so a "trade value" might be $600 if it had been shoot a couple times and well cleaned - any real signs of use would drop that to minimally $500 - and only if the dealer had an active market for black powder guns.
This is why private sales are popular for the owner, they will easily get 20% more than they would from a dealer on an outright buy or trade.
Whatever was paid in sales tax has nothing to do with the value of the gun - that was the original buyers "cost of acquisition" - so at least to a dealer that's not a factor in what it's worth - the government got that money, a dealer can't recover it.
And I would suggest that guns that are in low demand sell for "dealer buying prices".
Second, being black powder. If the owner was tardy about cleaning it problems have already started. That further devalues the gun.
Third, the market for black powder is small to start with - a used gun has even a smaller audience unless it's an uber-great deal OR it's an original or super rare and "somebody wants it".
The greatest drop in price with a factory made black powder gun will occur between first and second owner. The drop will not be that dramatic between second and third - almost all of the depreciation is built into the first sale.
Anyone buying modern, factory produced black powder guns better plan on owning and using them for years. They are not something to be bought and flipped after playing around with them for a summer. Serious black powder shooters won't pay a premium for a used factory gun. Guys/Gals just looking to "check it out" likewise won't pay a premium for a used gun that they may not ever develop an interest in shooting over the long term.
That's why Traditions makes a muzzle loader that sells for under 200 bucks - something to "try" and if you like it, you spend money on a decent gun knowing, that unlike a center fire starting/entry level, you are probably not going to get half your investment back if you choose to sell it.