The ammo for the .280 Ross was not belted, friend, it was semi-rimmed with a long, tapered casing: excellent feeding and all the advantages of rimmed-cartridge headspacing.
Loaded ammunition has not been available since about 1935, if you are looking at North American stuff. The British loaded it up to about 1966, but it was Berdan-primed.
Now that your heart is broken, I can send some good news: it is an easy cartridge to make. You start with .300 H&H Magnum brass and chuck the cases into a lathe and turn off HALF the thickness of the belt. Now you full-length size the stuff, trim to length and load it up. On firing, the case body will expand concentrically and fill the chamber, thus sealing it. As an alternative, brass IS manufactured in Australia and it is available here, just rather horridly expensive. Yes, I'm cheap!
You can load the cartridge with standard .284" bullets. There is a legend that the .280 was loaded with a .287" slug, but the only cartridge I have found which conforms to this is a single round of .28 Ross experimental ammunition. ALL of the original .280 Ross ammo I have (including a full box of King's Norton, made for Gibbs before the Great War) has bullets in the .283" to .284" diameter.
Ross used his own form of rifling in these rifles and it stands up very well when you consider that much of the old ammunition was loaded with highly-erosive varieties of Cordite. Just to be on the safe side (not wanting to waste powder) you likely would be best to stay with a flat-based bullet. Flat-based bullets obturate better than boat-tails and the upset will seal even a worn bore.
Ross loaded this cartridge extremely hot, all the way to 3000 ft/sec with a 180!!!!! Yes: hotter than a 7mm Weatherby! But your rifle now is very close to a century old, so I would stick with the loads in Barnes's COTW. Barnes recommends 60 grains of 3031 for 3000 ft/sec with a 140: pretty darned close to a good load for a 7mm Remington.
BTW, a commercial Ross .280 HAS exactly the same problems as regards correct bolt assembly as does the Mark III military rifle, so BE WARNED.
There are a couple of other threads on the go right now in this forum which discuss this.
As to value, I can't say. All I know is that I really want one while I'm still in shape to take it to the range!
You have a Very Nice Toy. Enjoy!
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