Hey @fototrop,
and thanks a lot for sharing your experience!
Indeed, right now with Sharetribe (both Go and Flex), we mostly focus on marketplace for services and rental experiences.
Product marketplaces offer great opportunities too, and shipping + stock are key for such idea. We hope to be able to better support them in the future, however I don’t have an exact timeline to share.
But the good thing is that with Go and the Community Edition, this is something you can customize at your own pace! Of course that requires a bit of time, technical skills and investment but you don’t have to re-do everything else 
I know that many others have customized Go to better support their product marketplace idea. Hopefully others will be able to share their solution and open-source their own customization to help you move forward faster!
Keep up the good work 