I was playing through yesterday and managed to approach Enthir at the mage's College in Winterhold. He told me he could sell me hard to get items, and sure enough he did. I bought several Daedra hearts from him (which he restocks at 2 every 48 hours), and went off to build myself some new armour. The problem is this. Due to a save/load glitch I lost a couple of hours work yesterday and need to do this again. This time though, Enthir won't offer to sell me anything at all. He just keeps telling me what I can't do while at the college. Does anyone know why he won\t sell me anything as he is the best way to get Daedra hears without waiting for them to be randomly dropped.
From there, it seems you cannot speak to him if the quest The Staff of Magnus is active.
Is this the case for you?
I was not the archmage yet, in the middle of the staff of magnus quest. He will not even discuss anything other than the rules outside his room. For some reason, once you are embroiled with the college in this quest, he won't sell outside of his room either.
However, once he enters his room a "can we do business?" option is unlocked. Follow this conversation line and he will sell.
If you became arch mage before talking to him, then from what I can see on the great internet you are going to have a lot of trouble getting him to sell.