It's the 2-handed Axe right? It's worth a bit more than the price you bought it for, probably an additional 250-300mil depending on server. The guy probably sold it at that price because no one was buying or even offering for it, since it is a 2-handed Axe.
I bought the Lionheart 2-handed Mace for the same price you did (150mil), and resold it for 500mil. It takes patience to resell unpopular items.