I see you specifically mentioned sidelock. Well, no kidding. A side lock large bore H&H, Purdey, Rigby, WR, etc. are quite rare to begin with.

Here is my thought on high dollar guns. When we start talking this class of rifle, either your a loaded and can afford them, but as an investor the liquidity of a $50,000 rifle is highly depended or what you have. You better hope you got it right when you purchases it.