So I completed Osmond's Request and Enthir still won't sell me anything. No dialog option pops up for me to buy anything from him. I'm trying to find some Deadra hearts. Anyway to fix this?

You must trade with Enthir before you become an Archmage or he will not trade with you.

Several bugs have been noted with the dialogue, but the default behavior indicates that if you trade with him before you become the Archmage, then you should continue to be able to trade with him.