Make sure that you have ticked the box to Add Sales Tax / VAT in the settings of each taxable product. If you don't see that option while adding/editing the product, look to the very top and tick the box there to Show Advanced Settings. If you want us to calculate taxes, you must also set up tax calculation.
Also note that our Buy Now buttons/links cannot calculate tax, as those skip our shopping cart to take buyers directly to instant checkout, leaving no chance to calculate tax in the cart before checkout. To calculate tax, you will need to use our Shopping Cart or Product Card purchase buttons/links, or sell from a Shop in our Marketplace.
Finally, if you're using PayPal, note that the PayPal checkout site initially will not show any tax/shipping amount(s) for orders that also require shipping, until after the buyer has provided their final shipping address, so PayPal can contact our system to recalculate tax/shipping based on that location. This ensures proper tax/shipping calculation and prevents confusing or misleading buyers in case their final shipping address does not match the country and postal code they'd specified in their cart to estimate tax/shipping before checkout, which may change the calculated amount(s).