The purpose of a paintball gun is to fire paintball pellets, to mark opponents in the game of paintball.
A paintball marker is a paintball gun. It is just called a marker instead of a gun.
typically yes, but its a good idea to call the police jurisdiction in the area as ask them first, and make sure that at any point on your field a paintball can not be fired and reach a public area where anyone outside your game could be hit or their property. if near houses the police like you to go to the closest houses and ask the neighbors, this is also a nice to do so they know what all the noise is
No, do not expose your paintball gun to water.
not often but in rare cases a luky shot might
It depends on what you mean by "sell". If you mean put it on Ebay for trade, or on PBnation then no. you have every right to do so. If however, you mean to open a shop and sell goods then yes.
Charles Gaines, Hayes Noel, and Bob Gurnsey played the first organized recorded game with people shooting paintballs at each other. There are stories of ranchers and forestry workers shooting them at each other, but none of the stories are verified.
A paintball gun is several magnitudes stronger then a Nerf gun. A paintball travels at around 200 mph, where a Nerf gun is around 10.