Problem I have with alot of Denner's early stuff is that I keep seeing pieces in person bought by ppl I know - many are in the US and UK. A good many of his pieces will have some degree of restoration on them and they aren't always advertised that way.
I'm sure Denner isn't trying to screw anyone, in fact it's likely he didn't even know about work on the guns before he got them, but when buying these high priced antiques, often $5000+ for the right Bess or Baker, I'd certainly want to know it was completely original and not, for example, converted back to a flinter from a percussion like one of his Besses I saw a few years ago. Granted, unless you took the lock off you'd never know, so likely JD didn't know it was a restore when he sold it.
Caveat emptor with high priced antiques!!! And I don't mean to single John Denner out in particular - he's as good a dealer as any other - it's just that ALOT of early, rare, and expensive guns are dinked with long before they ever surface on the retail market and a buyer can lose ALOT of money if they are ignorant about a correct piece vice one with issues.