Again, I play on a low population server. This makes keeping my stock up somewhat hard sometimes. I just bought 5,000g worth of Elementium ore last night, about 60 stacks (between 80 and 90g each). I don't really use the Snatch feature of Auctioneer, although I should set one up. If you do use it, great! If not, you should really learn; it makes buying a ton easier.
This evens out, however, with my decreased output of gems and other materials. Having transferred to a new realm near the start of an expansion, it's been hard to get a lot of professions going on my new realm. I'm currently leveling a hunter who will be my tailor and either scribe or blacksmith and engineer; I know that Inscription outclasses Blacksmithing by a ton when we're talking efficacy of making gold, but I'm also toying with the idea of getting my warlock over to Suramar, who is both a scribe and a tailor, but I digress.
This being said, make sure that you search regularly for good deals, and don't neglect using your bank alt(s) and get a bank guild (or more than one). Altaholic is a great addon for this; it will show you what you have on each character and in each bank, so you won't have to log on and check it out.
Space is virtually limitless if you don't have a ton of characters you play (like on my new realm, there's 4 characters that I play somewhat regularly - my shaman, hunter, rogue and druid, this leaves 6 bank slot characters), so be sure to utilize it to snatch up any stock you can find. Prices are becoming somewhat more settled, so the deals you find today will most likely be good when you need the materials in a couple months. Don't be squeamish!