To add VAT into your listings you have 2 options.
The first is to simply price your items inclusive of VAT. The downside to this approach is that your listings will appear more expensive to customers outside your tax zone where VAT would not be collected.
The second option is to set up a “tax policy”. This will add your VAT rate at checkout to any customer where VAT would be applicable (i.e. the European Union). If you chose this method be sure you price your items exclusive of VAT or else it will be added twice.