you need to look up 'thrall', maybe even read the book 'the thrall of leif the lucky' which you can read online.
although most, if not all vikings would have considered themselves bound by ties of allegiance and loyalty to some figure of authority, whether a local chieftain, a regional overlord such as a Jarl, or a king, and although slavery was an accepted feature of viking society, still they had a very strong tradition of a free class of common men. they were though, traders, they sought to increase their wealth through plunder, through taking captives and exchanging them for ransom or selling them as slaves. they did have servants caled praell or thralls (unfree bond servants) the lowest recognised viking class citizens who could eventually buy their freedom.
the reality was that the vikings' most common trade item was the slave. raiders brought their prisoners to the block, merchants bartered for sturdy or exotic breeds from as far away as serkland (the arabic peoples) or byzantium, while others were sold into slavery to pay unredeemed ransoms. nor did the vikings confine their raids and slave taking to peoples outside the scandinavian countries: at this time, there was no sense of nationality in the north. icelanders had danish slaves, the norse kept swedes, the swedes captured finns. hereditary slaves were treated no differently in the laws from new captives.