I'll give you $100 for that rusty old thing. It's worthless!
Just kidding. You are in possession of a great piece. Working Sterlings fetch a 5 to 10k price (depending on the condition of course). Based on your pics I'd say you have fine gun there. Probably in the middle of the range mentioned. You can search for "Sterling Mark 4 gun auction". That's going to give you some comps. I believe I've read somewhere that most Sterlings where decommissioned and destroyed. So I think one is that condition is highly sought after.
I'd also add, don't do anything to it if you want to sell it. Collectors are going to want it in the condition you have it. Don't sand it or paint it.
Hope that helps.