@Lepidopteran Thanks for highlighting one of the major problems during the time of the pandemic. The FCCG of W3C looks interesting.
As far as I know, federated e-commerce isn’t a solved problem yet. I didn’t even see vaporware out there that claims to provide a federated view of multiple shops to a customer.
In my opinion, this is a problem in need of a social solution like a cooperative of multiple shops in an area selling through one unified e-commerce portal rather than a technological solution like yet another federation protocol. It can be like Amazon marketplace but without the exploitation from the centralized big player, and more localized to an area.
I am aware of Open Food Network. We considered using it in rural deployments of FreedomBox in Indian villages (but didn’t go through with it). It can be used for decentralized e-commerce, i.e each shop hosting their own web shop, but it is not federated. We imagined each farmer coop running one instance of Open Food Network.
WordPress is one of the applications that is being considered for inclusion into FreedomBox. There is an e-commerce plugin for WordPress called WooCommerce that can be used to run a small web shop.
We should also keep in mind the fact that FreedomBox is not intended for large scale operations out of the box (though a talented system administrator might be able to scale it by provisioning more hardware). I find it easy to imagine a self-hosted replacement for Etsy (but with one seller) running on a FreedomBox Pioneer device. Maybe it can also handle a low-traffic store. 
Sometimes, we tend to gravitate to high tech solutions to most problems. Today, I made a purchase by sending my address as an SMS to a merchant’s phone who sells only one item, so he doesn’t even need to know what I want to order or a storefront to display his one ware. I paid in cash on delivery. 