Sellback
As with any other nonstory essential item, cubes can be sold to any merchant in the game. Their value to merchants are negligible, earning players a measly 25 gold. However, selling cubes is a good way to free up space in your inventory if you are carrying too many.
Swindle Bilk
Swindle Bilk is a nonplayer character in the secret caves underneath much of the "AdventureQuest" world, called Tercessuinotlim. Swindle has a wheel of chance that he spins; when it stops, it determines which of his vastly powerful items he will give you. To earn spins of the wheel, you must complete quests for Swindle. But to even enter his cave in the first place, you must pay 50 cubes to the NPCs outside his door.
Corpse Maker of Miltonius
The Corpsemaker is an extremely large, one-handed long sword made from steel, human bones and crystal. The crystals of the blade glow with a light that you get to choose the color of. What makes the weapon notable is that it is extremely striking and immediately noticeable. You can acquire the Corpse Maker either by completing a complex quest or by using 50 cubes to speak to Swindle Bilk.
Ddog Sea Serpent Sword
The Sea Serpent Sword is a bulky short sword with spikes on the handguard. The blade rises out of a dragon like sea serpent's mouth and appears to be made out of fine blue steel. It can be acquired in one of two ways: from the same quest that will give you the Corpse Maker, or as a reward from Swindle Bilk's wheel of chance.