I'd love to know how stores are synchronizing their entire shelf+backstock inventory to Craiglist or FB Marketplace given neither service even has an API (Craiglist allows bulk posting but that's useless if you can't bulk delete/update for availability) or even basic metadata like product quantity, UPC/EAN, etc.

Ignoring that, the services you're talking about don't even do any kind of canonicalization or grouping of products like Amazon, so if you search for "Xbox" you're going to have to click through many pages of individual listings rather than having a unified product page displaying product details and specs from the manufacturer, consumer reviews, related product variants (e.g. color), etc.

It's fine if you think the idea's weak but at least give the idea a fair shake before dismissing it.