I did not see that you were looking at a used "black powder". Be darn careful. Properly done after shot cleaning is an absolute must when using any of the black powders.

Take a really careful look at the rifling, breach plug fire hole, barrel crown area, etc. ... looking for signs of corrosion and/or rust damage. Ask to demo the rifle to see if it groups well. Corrosion damage to these areas will definitely show up as poor grouping.