Does it say “Loading Preview”?
Also, you already refreshed your eBay access token and the eBay category data, right?
You should make sure that your network does not block the connection (Firewall, proxy server) and that you don’t use a tool that could block the network (Kaspersky, LittleSnitch, MacKeeper, HandsOff, cisco anyconnect, GlimmerBlocker or any other AdBlocker).
If you create a new listing from scratch using the “+” button in the toolbar, does it work then?
The problem appears to be with the GarageSale Item Attributes [[item.SKU]]. I replaced the brackets with braces {{item.SKU}}, but this did not solve the problem. I’m unable to use the code above to insert the SKU into the listing description.
Has something changed in v7.0.9 of GS that prevents uses of Item Attributes?