50-350 usd
I would start my search at Smith and Wesson first.
Looks like any other pump shotgun. Usually one or two on the auction sites with pictures.
The markings.
500$
yes
The Mossberg 3000 was manufactured in Japan, and licensed from Smith & Wesson, who in turn licensed it from Howa of Japan. After 1995, it was no longer imported into the US, although it still remains in production.
Best left to a trained gunsmith
50- 325 USD or so
100-300 usd
You must call S&W for a correct answer.
You should call S&W to get the correct answer from them.