If you have the equipment, yes. Although you may want to ask if it's legal.
No.
As long as you shoot it on your property at your property.
If you only shoot your property, on your property you will be fine.
No. Because, in order for you to shoot them you would have to get their permission to shoot them. If you accidentally shoot somebody inside your backyard without their permission, it is their fault for being in the yard with someone that has a paintball gun and is shooting it in the first place.
Yes it damages property so it is vandalism. Vandalism isn't always a felony, plus shooting a paintball gun isn't necessarily illegal. It depends on the circumstances.
its a marker, not a gun. So technically if there are gun laws they don't apply to paintball markers. Now if there were laws against painting, then no you couldn't. But then again the paintballs have biodegradeable stuff in them and are not considered paint. Are you trying to ask if you can expel dishsoap in your yard with a dishsoap marker? That's what it sounds like to me.
open carry in your back yard in massachusetts
yard sale, ebay, craigslist, etc...
about 15 yard.
If you shoot on your property at your property you will be fine.
Generally online only, unless you find one at a yard sale or flea market.
Depends entirely on where your back yard IS- and the local laws.