If you can sell them, by all means, sell them for what Sinister has said, and good for you! I've just never seen anyone pay any price for magery or use it. Some of the masteries are really good, but when you can only have one active, magery has never been the choice over spellweaving or taming or several others among the folks I play with regularly.
Necro sold well at first, and I pointed out that it can be useful, but since those first few months I have run into exactly zero folks using it. Didn't mean people don't use it, just means I've never seen any evidence. If I'm going to pay 5 mil + for a mastery, it needs to be something that I will use every day and it needs to be something that fundamentally improves my game play. I've never seen that with necro. Maybe others do.