Can you buy a gun online?
Yes, you can buy a gun online, as long as you can legally own a gun. You must comply with all state and federal laws. Once you purchase the gun, unless it qualifies as an Antique, it must be shipped to a local Federal Firearms Licensed Dealer to legally transfer the gun to you. Here is a great site to buy firearms http://www.gunsinternational.com/ Great Gun Classifieds site, over 25,000 guns for sale from Dealers and Collectors.
Well, depends on what you consider "good". Auction Arms, Gun Broker, and Guns America are all sites which function similar to how Ebay does, except with guns (Gun Broker is actually a subsidiary of Ebay). Many manufacturers and gun shops also offer an online store. Just remember that when you buy a gun online, you'll still have shipping and FFL receiving fees on top of what you paid for it.
In the US you have to have a Federal Firearms License to be able to buy guns and have them shipped directly to you. People who want to buy a gun online make arrangements with a licensed dealer and have the gun shipped to the dealer's shop. Dealer then performs the required background check and transfers the firearm to the buyer. This service typically costs anywhere from $10 to $50.