Some Norsemen did not wish to work for a trader or to become a merchant. They had no interest in farming or talent for making quality goods. Instead, they tried to get rich by looting. Raiding parties were composed of about 100 Viking men. By the time the people in the foreign countryside, town, or village they were looting had gathered together a group of men to fight them off, the Vikings had sailed away on their speedy longboats.
It was not against Norse law to loot foreign villages or towns, so anything Viking looters got away with could be sold or traded in the marketplace back home. But it was against Norse law to steal. Norse law was clear - to the victor goes the spoils. If people did not know they were being robbed, that was stealing. (To steal, to sneak, to not take honorably.) So, on their way out the door (so to speak), if they did not have to fight to get away with goods, the Vikings would set the building they had just looted on fire. That not only kept the locals busy putting out the fire rather than chasing them, it also announced their presence, and thus, to a Viking, was not stealing.
Whether they traded or looted, it was the power and speed of the well built Viking longboats that gave the Vikings a decided advantage.