The Empty can of whoop-ass is an artifact.
It is the result of a previous hero unleashing an fatal attack using a combat skill. With the odd physics of Godville (Game), it appears as it is said: "I'm gonna open a can of Whoop-Ass on you!" Usually this can is vaporized upon use, but occasionally, it is left behind by a hero that just defeated a monster. Because the can is empty, it initially isn't worth much. Its value is increased simply because the can is metaphorical, and it shouldn't physically exist. It's also rare to have one that is still intact. Traders may give the hero 15 to 200 gold coins for this artifact, depending on the hero's aura, level, and the town they're trading in.