Is your skill level high enough to buy Adept level spells?
I read in the wiki that you don't need a particular skill level to buy the spells. Is this not true?
As a test, I quit the game and reloaded and the conjuration spell merchant suddenly had the adept level tomes available this time, but the other merchants didn't. It sure seems like a bug.
edit: my highest spell skill level is conjuration at 40, destruction next highest at 33.
In those few minuets before going to the college, did you do any quests or level ups or change the game difficulty?
I had leveled, but I saved the game, then reloaded from that save, so it shouldn't have been an issue.