You didn't mention whether it is flint or percussion. Flintlock rifles generally command a higher price than percussion. If it's a good clean rifle with minimal usage marks and a good bore... offer $300, maybe a bit more if it's a flinter. But first, ask the seller if you can run a clean patch or two in and out the bore, that will give you an idea of how it's been taken care of. If the bore is rough or the patch comes out rusty I would pass on it