On the EU website Your Europe you can find some practical advice about doing business in Europe. On the website you can read about what laws, regulations, business entities etc. are applicable for respective country.
Customs declarations and payment of import duties are not required when trading with one of the 28 EU member states. However, you do need an international VAT number. When you trade with an EU country, the Swedish VAT number is prefixed by "SE" and "01" is placed after the number. In the EU, VAT is paid in the country of consumption, namely the country in which the product or service will ultimately be consumed.
However some exceptions exists, why it is a good idea to familiarise yourself with the VAT rules before you start trading in the EU. Please read the VAT Brochure from the Swedish Tax Agency for more information.
Go to Your Europe
Go to the Swedish Tax Agency