Provided that the bulk of my sale list is Heroclix, is it ok to add a DVD Box Set onto my list?

Just thought I would check before adding it?

I think that's fine. As long as most of your stuff is 'Clix, I don't think there'd be a problem. Line 15 of the new rules does read:

15) This forum is for the buying, selling and auctioning of heroclix

...but it doesn't say that you can't list anything else. I've seen it done plenty without any repercussions, at any rate, so I wouldn't worry about it.

A few non-Clix items here and there is fine.