^^ What they said.
They usually call 'em mules 'cause they were used by minishop owners, so the actual person's account wouldn't get flooded with trades and PMs.
And since it did all the work, like a pack animal...mule.
An account someone uses to store surplus gold/items on websites like this, so, say for instance, their main account gets banned or hacked, they won't be starting over from scratch.
Though, that's not the only reason people will have mule accounts.