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.
No
100 USD
50-125 or so
100-300 USD
Follow the instructions from the manufacturer.
Try e-gunparts.com
http://travel.webshots.com
Blue Book of Gun Values will help.