In the photo, the work looks well done if not original, and if the gun is in good condition, it's surely worth $300 as a hunting gun. Not many people collect S&W long guns, anyway. Have you priced Rem. M-1100's in comparable condition?
If the work is aftermarket, you might ask a COMPETENT gunsmith if it needs a clear surface protectant.
I think you should send the photo to S&W and ask if it was made this way. If so, I think you got a bargain. I'd look for some key spare parts, including rubber gaskets while they're probably available. I'm guessing the gun uses those O-rings, like the M-1100?
Is this receiver steel or light alloy?