Yes, there are as well as people who collect other types of cartridges. Both the loaded type of shotgun ammunition and the empty 'hull' of a shotgun shell. If it exists, there is probably someone who collects it.
Most commercially available shotgun shells come in boxes of 25.
Air soft shotgun shells are the size of 12 gauge shotgun shells.
Find someone who reloads shotgun shells. They will relieve you of your inventory.
Try Googling 'shell collecting' and see what you get.
No
You can use nothing but 410 shells in a 410 shotgun.
NO
Shells
No.
Shot and Slug
Yes, you can.
Generally "Pellets" or "Shot" contained in "Shells". There are also solid shot "Slugs" similarly contained in "Shells". I'd use Shells ie "8 shotgun shells", unless you are specifying exactly the type.