I voted that I buy all of it, but that's not necessarily true... it depends.
I shoot a few odd cartridges... .41MAG, .348WCF, even things like .45-70, which no one leaves laying. I prefer to buy all of my own brass, or source it from my once-fired factory ammos.
Unlike some here... I'm really, really picky about what I pickup at the range and actually use. It is a rare event that I will keep range pickup rifle brass, and this goes back to my misfortunes with 'once-fired, processed' military brass in 7.62mm.
I will pickup 9mm and 45ACP range brass... depending on the headstamp. I'm really picky about what headstamps I load... so if it's something I like, great, if not, it goes in the scrap bucket. I do not pick up brass I don't reload for... 38SPC, for example, or 40 S&W... I just leave it lay.
I will buy brass in as big a lot as I can. I'm still shooting my RP .41MAG brass that I bought in a 1000cs lot back in the 90's, for example; same same with some .45ACP brass, although that is pretty tired brass and I'm starting to retire it. With something like 9mm, when I get low on brass, I buy a 500 or 1000round case of factory ammos, shoot it, and then I have more brass to load. I do the same with 7.62mm brass for my M1a.