Depends on the laws where YOU live- and you did not tell us where that is. In MOST of the US, it is legal for a private citizen to sell a firearm to another resident of their home state, so long as you have no reason to believe that person is barred from owning a firearm.
Yes. Try the auction sites or adveritse in Shotgun News, Gun List, local gun store, gun shows.
Newspaper, gun show, gun shop, Shotgun News, etc..
Pawn shop, gun shop, gun show, for sale ad
Gun shop, gun shows, want ads.
It is legal if you have permits for the gun and only in certain areas.
Legal age to buy a rifle or shotgun is 18. 21 for handguns.
Gun shop, gun show, pawn shop, for sale ad.
The National Rifle Association promotes safe and legal gun ownership and represents gun owner interests to Congress.
You can sell it at a gun show, you can sell it through classified ads (if your state permits it), you can sell it on consignment at a gun shop or sell it to the gun shop, you can sell it online at websites such as Gun Broker/Auction Arms/Guns America/Cheaper Than Dirt Classified ads, or you can post "WTS (Want to Sell)" ads on forums, or you could even sell it at a garage sale if you so wished.
You can sell it to a gun shop or pawn shop (or sell it on consignment through either one), or you can do a private sale.
You can't. You will have to sell your old air gun, then use the money towards a new PCP air rifle.
sorry buddy but its illegal