@bart
If you just want a gallery, Pixelfed is a pretty good existing AP option.

e.g.: px.filmfreedom.net/lunatics?m=

Is it the attachment to the print/merch storefront that you're looking for?

I've given this a bit of thought -- enough to decide it's beyond me.

But basically, my notion is that instance-hosted storefronts could have federated catalogs and carts, but process transactions locally. Resulting in something like "Ali Baba" or "Amazon Marketplace", but federated.