There are several types, you would either be interested in a regular dealers (Type 1) or a C&R (Type 3).
The C&R is what is traditionally thought of as a "collector's license", it allows you to purchase from dealers and have them shipped direct to you, bypassing a local FFL for transfers. However, this only works for firearms on the federal C&R list. Most ads specify if a gun is C&R eligible or not. You are not a business, so selling other than the occasional gun from your collection is not allowed, IOW no ordering a Mauser "for a friend".
The regular dealers license lets you buy and sell any type of firearm (except NFA) but it is a business license. You have to have a legitimate business selling guns, gone are the days of the home dealer who works out of his kitchen. ATF requires you met zoning laws, fire codes, etc. to get a dealers FFL.
Both licenses are applied for the same way. Contact your local ATF field office and ask for the license application forms. Fill them out, get fingerprinted on the enclosed cards, and send back with a check for the fee.