What is the rule for online persuasion again about using currency; should you use the dollarsign or not to get people to buy?

Not sure what rule you’re talking about. But I would say always show the users local currency.👍

haha, sometimes it is better to just show the number without the currency

Imo, I would strongly disagree with that! It causes confusion. The first thought from someone will be “100 what? Euros? Dollars? CAD?..” instead of “$100. Is this product worth that?” You want the second question.