I know a little about the percussion doubles market, and obviously, you have a nice one. The market is very small, and in the US tends to favor US type flintlocks fowlers, which is a totally different gun. Still, there are buyers (and if I wasn't totally broke, I'd be interested). Here are 3 web sites to look for comparison valuations:
Track of the Wolf, in Minnesota: handles a wide variety of used muzzleloaders from London doubles to Thompson Renegades. Prices are probably fair market retail for guns to use.
Roosevelt and Drake: very high end, with high-end prices; but they sell their stuff. In their market flintlocks seem to go for about double of percussion.
Vintage Doubles: Kirby Hoyt sells vintage doubles, and has a few percussion guns (they cover a broad range of prices). I bough a gun from him, and it was as advertised.
In my purchasing of antique guns (shootable) I have found that damascus barrels that seem very good on initial inspection can turn out to be considerably rougher when carefully cleaned out. Don't really know why, could be you just see it more, or it could be that what's in there fills in the rough areas. It's worth considering before doing a cleaning job on the barrels (or not).
