The Vikings raided when they could and traded when they had to. It's cheaper to steal something than trade for it.
More fun too.
They didn’t use longships for trading either - they used a knarr - http://www.cdli.ca/CITE/v_knarr.htm
The longships were warships and raiders designed to move quickly and slip over port-protecting sandbars without the need for a local pilot - I think they only drew about 3 feet of water which isn;t much considering how long they were.
“It’s cheaper to steal something than trade for it.”
- Only in the short term perspective.
The ideal is the situation we’re experiencing today; most parts of the world working hard and profiting from competing as well as doing trade with each other.