This is not a GCU issue, it's changes to Pinterest. They no longer allow it from many POD sites.
 
You need to bring up the Marketplace view, Save the image. Then upload the image to a new Pinterest pin and add/replace the link with your Sell This Card link.
 
Pinterest did this change because too many people posted via links without adding unique content, so it's clutter and clutter that Google won't index.