Another idea instead of "buying" spells is that maybe there could be an NPC that could teach you spells.
That's pretty much what "buying" spells is. You're paying someone money to teach you a spell, and that makes sense, just as you can pay someone money to learn how to do something in real life, there are people who make a living by teaching people things in Tamriel too, and spells are included.
And having all spells known from the start but needing to "unlock" them wouldn't make much sense, because spells aren't something you instinctively know but need to get enough skill to be able to use it. Spells need to be learned, like from someone else who has the knowledge. Certainly, there can be alternate ways to learn spells beyond "buying" them, like say, maybe some books could allow you to learn spells once read, as people might record the spells they know in a book, but having spells be known from the start but need to be "unlocked" wouldn't make much sense.
Not confirmed dude. Yet

The way Todd Howard put it, it doesn't seem to be so much "not confirmed yet" as it is "We don't have it right now but we're not going to rule out the possibility that it will be added later, still, it probably won't be." and honestly, I'm glad it's gone if it is indeed removed from Skyrim, magic was incredibly boring in past Elder Scrolls games, each spell was pretty much a generic formula, it had a strength, a duration, and possibly a radius, it could be cast on self, on target, or one touch, and really, that's about all, there were different effects that could do different things, but spells with the same affect never felt that distinct, one shield spell was exactly the same as another, except it may be stronger or last longer, you might as well just call them "Shield" and "Slightly Better Shield", and when you made your own spells, you were just making the exact same spells as the default ones, only with slightly different effects. Oh my! That's so impressive! Honestly, I never felt like a mage devising impressive new spells when I created spells, I was just changing some numbers on a spreadsheet. And let's not even get to attack spells, as whereas charm and chameleon and such each have a fairly distinct effect, there was absolutely NOTHING seperating say, a lightning spell from a fire spell with the same stats, except that it had a different looking projectile and different creatures had resistences or weaknesses to it. And mostly, I blame this on spell making, lots of RPGs don't have spell making, in fact, I can't name any game besides the Elder Scrolls which had spell making, and their spells tended to be a lot more varied and unique than the ones in Morrowind and Oblivion. If removing spell making will allow Bethesda to make spells more unique, than I won't miss it at all.