Well of course the Danes and also the Norwegians were great merchants and no doubt traded in everything of any value. Human slaves had high value. The typical slave bounty was 5 hundred dollars, I have read. So what do you mean by "slaves"? Was it indentured slaves from Ireland? Or, from many other European nations? Anyway, my ancestors from near the Arctic Circle didn't have slaves. My Grandfather was rejected from university because he only wrote in Landsmaal, the dialect of the far north. The whole family picked up and moved to America, I think it was 1887.

Of course, the Norse were not the oldest slave holders. The Romans, the Greeks, the Egyptians, the Sumerians, they all had slaves captured by force from conquered lands. Probably the Irish had slaves. It's a very old idea.