In U.S. firearm dealer license is called Federal Firearm Licence (FFL). There are nine types of FFL licenses ( nine types
http://en.wikipedia.org/wiki/FederalFirearmsLicense#FederalFirearmsLicensetypes ). In simple words firearm dealer license allows you to trade/manufacture guns and ammunition. Usually people hold home based FFL license to save money.
How much can you save?
Up to 30% on guns and ammunition! So instead of paying almost $1200 at gun shop for a Bushmaster AR-15 M4, you can only pay a little over $800! Instead of paying $1250 for a Smith & Wesson 500, you can pay for it just $800 and change! Among other things, when you sell or buy gun you don