Re: What makes a round non-reloadable?
Yeah, a lot of that stuff is Berdan-primed, which has two flash holes instead of one down the center (which makes it easy to decap primers). Some people (far more dedicated than me) do reload Berdan-primed cartridges, but generally it's correct to say their un-reloadable.
I know this is more of a "down-the-road" observation, but some brass is unreloadable because it has simply worn out or has been over-pressured (or both). Stuff that has been fired hot - and maybe it take a few times - will be more brittle; brass may have flowed out of the head area, making it weaker; and it may have cracks in the mouth and elsewhere. For the most part, there's nothing wrong with buying once-fired brass, but don't expect to get the same number of firings as you might out of brand-new Lapua, for instance.