@saab,
Many merchants were opportunistic vikings--if they saw a village they thought they could take, they'd come back just before dawn and raid it. They acted as spies, too. A merchant would pay attention to local gossip, and pass the word along when they returned to their home base. Haithabu (now called Hedeby) was not just a place to trade merchandise, but information, too. Although there is a romantic and completely unreliable explanation for the invasion of Northumbria in 866 by Ivar the Boneless and Ubbe Ragnarson, the most obvious explanation was that Aelle of Northumbria was facing civil war, and the Ragnarson brothers exploited the situation, which would have been widely know through merchant gossip.
But a lot of people think Viking sounds cool. I have a copy of BBC's
History magazine for the 2013, 1000 year anniversary of the conquest.n of England by Sweyn Forkbeard. The article is entitled "The First Viking King of England," and not the first Danish King of England. Viking probably sells better.