I have bought and sold multiple previous fired brass. Is not uncommon for 303 British to develop an "incipient head separation" on the very first firing, if the reloader does not know how to fit that brass to an oversize chamber - your No.1 Mk3, if WWI British, is about prime for having had the chamber made deliberately over size, when in service. So, need to inspect inside of the fired case - just above the solid case head - if it has stretched, will be a ring formed on inside of case - can also be felt with bent end of a paper clip wire, etc. Phenomenon is not nearly so common on rimless cases that headspace on the shoulder - like 8x57JS and others - but most any case that headspaces on the rim or belt can have that "incipient separation" after just one firing. I know of no way to "undo" that - to me, a case like that has no value for reloading, at all - maybe as scrap brass??
That separation thing is matter of degree - not black and white - some rifles will have case head separations on first firing of some "fresh factory ammo" - some may last for three or four reloadings. Is all related to how well or how poorly the case to be fired fits to that chamber - which can be grossly affected by the hand loader who incorrectly sizes the fired brass.
And, for certain, as mentioned - that cost to mail likely knocks the "sense" out of small quantity sales. (small quantity - like less than 100 pieces or so - unless very obscure cartridge or particularly interesting maker). In our small Manitoba town, can buy Canada Post Small Flat Rate boxes - so that pays for postage to any address in Canada - comes with $100 Insurance and tracking - weight is limited to 5 Kg (11 pounds) and lady at our post office very adamant that the box can NOT be deformed - must be the original size, not "bulged". I have received and sent brass multiple times in those - here, they cost $38.83 for two of them - I do not know if they are same price across Canada or not?
In some cases, Internet ad as "once fired" has evidence cases have been reloaded and fired several times - typically has had primers removed and tumbled - looks "good", but primer pockets very loose - is almost impossible to tell that until trying to seat primers. I have even seen posts on this website suggesting to just clean up and "sell" used brass, and claim it to be "once fired". These days, I tend to limit my purchases to "once fired" military brass - if it still has the crimped in / sealed primers - I am completely able to remove those primers, swage primer pockets and tumble if I want to. Is of no value to me for a potential seller to claim to have done all that - in my limited experience is also a way to hide or conceal previous use of those brass. On the other hand, going through last bag of 100 new 22-250 Winchester red labelled brass - factory new - I cut the bag open - at least 20 of the 100 had potential splits, visible folds, or case mouth deformities that prevented those factory new cases from chambering into my rifle. As mentioned to someone else, I never had that many deformities previously, in total, and I have been reloading since late 1970's.
