Big rifle like 30-06, .308, etc are worth ten cents each or more if premium headstamps, shipped.
.223, .30-30, I get about five cents each for shipped.
Odd rifle, stuff like belted magnums, hard to find calibers like 6.5 Swedish, etc are worth fifteen cents each and up plus shipping.
Common pistol cartridges, .40, 9mm, etc are worth 2-3 cents each shipped depending on tumbled or not, nickle or not, etc.
UNcommon pistol cases like 357 sig, 45 gap, 10mm, FN 5.7, are worth around 5-10 cents each shipped depending on how bad the buyer wants them.
45 acp and 38 special gets between 5-10 cents each also, but it's not "uncommon" it's just in demand, again, depends on headstamps, nickle or not, tumbled or not, etc.
Uncommon revolver cases or anything Magnum is worth at least ten cents a case plus shipping: .44 Special, .44 Magnum, .357 Magnum, ,45 Colt, etc.
LARGE pistol cases (500 AE, .454, .460, .480, .500, etc) I get fifteen cents each for them minimum, plus shipping.
With common stuff, the key is to have bulk quantity, because nobody wants to pay $5 to ship in 500 cases. They want to pay $10 to ship in 3000 cases.
USPS Flat Rate Box is your friend (although it is now $9.30 per box instead of $8.95)
eBay terminated sales in brass, projectiles, or any part needed to make a gun fire mid-2007 as a result of the Virginia Tech shooter buying a magazine on eBay.
AuctionArms or Gunbroker or this forum's Trading Post are good venues.