Proof of ownership documents, along with proof of sale (proper Sale Invoice with valid VAT number from the seller).
This is the HMRC guidance for UK businesses making VAT purchases in other EU member states -
http://www.hmrc.gov.uk/vat/managing/...as-traders.htm
Therefore, it seems the refund scheme may also apply to you in Sweden. I imagine you pay the VAT here and reclaim it through the scheme using your Swedish VAT registration. Otherwise it seems the acquisition rules may apply:
"You can't use the Refund Scheme to reclaim VAT on:
goods you bring in from another EU country (these are called 'acquisitions') ...
... When you bring in goods from another country in the EU to use in your business, these are dispatches from the other EU country. You can't use the Refund Scheme for them but they'll be zero-rated, as long as:
- your EU VAT number is shown on the supplier's invoice (for UK VAT-registered businesses, this is your UK VAT number)
- the supplier has evidence to show that the goods have been sent."
Obviously, these are the rules for UK traders but I assume there is some generalisation of VAT rules across the EU. I would check with your local Tax office for advice on the scheme mentioned.
In short, pay the VAT and claim it back or have documents to show that the seller is VAT registered and has supplied the goods in question.