Sorry, Clanad, I've never heard that. Some stores hold "red tag" sales or "blue dot" sales or use other such names for markdown prices when they use a sticker to indicate on the price tag that a standard discount will be applied at the register.
But as Ouisch says, a tag sale is a yard sale or garage sale -- held at a person's home.
What you call jumble sales in UK, we call rummage sales -- many people donate to a church, a school choir, or other organization, which runs a great big tag sale as a fundraiser for its worthy activities.