We've been using BigCommerce for about 3-years now.
At first our products were ranking in the first page or two of Google and sales were very promising and on the rise.
The about 18 months back something changed and we lost ranking and sales. We also tried spending around $600 on Adwords but the only winner was Google.
At the moment we are barely covering fees with sales.
So I'm currently engaged in updating all our product images. As I go I'm updating eBay listings with the new images. But when I'm done my intention is to migrate all our products over to Shopify and give that a try.
I will buy a theme from the Shopify store.
We also did this with BigCommerce but the theme had never been updated by the developer so wondered if this might be why the sag in rankings.
Anyway a change to a whole new platform and theme might do something good.
I guess in about another month I will have all the images redone and be ready to launch on Shopify.
Good luck with yours.