You'll want to look for dark spots or streaks in the bore. Check the condition of the rifling in the area right in front of the chamber for wear, that gets especially hot. Copper build up isn't necessarly bad (although it will have to be removed ), but it at least tells you that the rifles been used. I personally wouldn't buy a rusty bore, but other more knowledgable members could probably clean that up ok. A little bit of pitting or corrosion might not affect accuracy, but its still something you'll always have to watch.