Federally, a 03 FFL (Curio & Relic) allows you to build a collection of qualifying firearms through interstate commerce without relying on 01 FFLs (Dealers).
In other words, if I want a C&R gun, I can buy it and have it sent right to my house.
Generally, it is more to facilitate buying than selling- you still are not allowed to sell for profit (this can be nebulous, but so long as you aren't buying and then selling a bunch of guns in a short time you'll be OK) or transfer guns for others (no buying C&R guns for buddies). Sometimes if you are buying locally, a dealer will make you fill out a 4473 instead of accepting a copy of your FFL (the transaction can be handled as a transfer between FFLs, not as a sale to a regular Joe); this is dependent on the policy of the dealers themselves. Not all of them like the fact that we can cut out the middleman on the milsurps. The C&R FFL is of most benefit when buying from distributors and taking delivery at home, since you can get those guns for the same prices as the 01 FFLs.
Unofficially (the ATF has nothing to do with this), you can be considered a "dealer" by certain accessory retailers (like Brownells and Midway USA), so you can take advantage of dealer pricing. But that is more a side benefit than anything else.
