A lot depends on the type of things you are selling.

I mainly sell clothes and find that I am better off selling new items, especially if I have more than one of the same thing, as store items but would not consider it unless I had a minimum of 50 items to list. I do have a few auction items runnning at the same time just to keep at least one item near the top in ending soonest searches.

For most other second hand items I use a different account that qualifies for the 30 free listings a month. So far that has been enough but I have other accounts that could be converted to selling accounts if I want more than the 30 free listings.