As has been highlighted, Oztion won't be bringing you that much traffic so you should never expect sales to chart very well on the platform. Having said that, it is a free listing site and therefore all your are wasting is your time by being there. If you consider that to be an acceptable use of your time, then I suggest keeping a profile there for sure.
eBay does cost of course, but it delivers traffic, and that's what gets sales at the end of the day.
The trading post also operate an auction site, although I don't think they have free listings and they don't yet offer an auction platform on all categories, for some completely bizarre reason that is lost on me!
One other point I would make with the product line, I would suspect that sales in general would be down a little for the Australian designer sunglasses market during the winter period. No, I have no stats to back that opinion up, just my way of thinking, so that could also impact on sales no matter where you are selling : )