I hope it's not like in Oblivion where you 'bought' a spell like it was a sword or shield, I'd understand if you were buying a tome or scroll to learn it from, but it makes no sense to seemingly buy the knowledge itself and instantly learn it. Spells are sold as spell tomes, so you don't buy the spell, you buy a book that teaches you how to use it. Source? I know you can get spell tomes but I didn't know you bought them. If so, that's a massive improvement. I'm afraid I can't link the source; it was from a walk-through that has now been removed until the 11th. You can find the tomes as well as buy them, apparently.