You have stated what you have, not asked a question.
a horrible gun
Good shotgun
The value of a Nitro Marvel 1886 16 gauge shot gun in working condition is between $199 and $165. As a parts gun only it is valued at $39.
$50 more or less.
100 USD
No
50-125 or so
http://travel.webshots.com
100-300 USD
Try e-gunparts.com
Follow the instructions from the manufacturer.
Post the serial number of the gun and I'll let you know.