Maybe they could have certain items require an active production source to restock, so you wouldn't be able to buy ore until you clear the Renegade Conjurers out of the mine and deal with the bandit fort hitting the trade caravans. That could make mines have more of a use and make it more challenging to obtain ore, but not necessarily making you mine it yourself. Mines could occasionally be overtaken again, but if you're a high rank in some orgainization you could assign some guards.
Also Ebony's coveted stuff and the city rulers would want to reserve it for their own military, so maybe you'd need to be a Thane (or the equivalent) to be allowed to buy the best materials, otherwise you'd be getting the leftovers.
Some old-school RPGs actually did things like this, however they shot themselves in the foot at the same time as there were only ever a handful of uses worth bothering with for said rare resources. It's an age-old problem that doesn't really have any good solution(s).
More OT: while I prefer mining to buying for collecting smithing resources, I often end up doing both as it's the only way to actually sell all the items I craft, so I wouldn't want to see the 'purchase' avenue closed off.
@wtflag: some older games had the 'chance to fail when crafting' mechanic, which worked well to prevent excesses at low(er) skill, however I'm not sure it would work in this instance. In older games, failing a craft meant you had just burned a decent portion of your supply of the rare resource said item required, which served as a brake on 'spam' tactics because it was hard to get more; when said item is limitless, however, you don't care how many times you fail because you can always go mine and/or buy more.
Additionally, given that money is meaningless in
Skyrim beyond use in the purchase of more materials and occasional training, there's no real financial cost either as a handful of well-enchanted items will more than cover the losses incurred by the failures. Basically, all such a system would do is screw over those doing their low-skill crafting 'normally', which won't please anyone. Which is, honestly, unfortunate, as there's real merit in such systems.