Three kinds, three formulas
VAT sits inside the price: a €120 item at 20% contains €20 of tax — one sixth of the price, not one fifth. Turnover-style taxes take a plain share of the price. Sales tax in the US is added on top by the marketplace, which also remits it, so it never leaves your pocket.
Get the kind wrong and you either give away three percentage points of margin or price yourself out of a market for no reason.
Below the threshold
Small sellers are often exempt: the small-business scheme in Germany, franchise en base in France, the registration threshold in the UK. If that is you, the honest setting is no tax at all — and the price you see is the price you keep.