Used powder and primers are not worth messing with
Would it be possible for you to explain why?
Aside from any legal issues of selling black powder and percussion caps, would you buy those items used not knowing the age or storage conditions they have seen? Heated dry storage is good, bubba's outdoor garden shed with the leaky roof is not so good.
As far as all the small packaged parts you could try evil bay, but it's already loaded with that kind of stuff and not many bidders. Gunbroker is even worse. A local gun show would be the best bet IMHO. Be prepared to sit on it for a period of time.