For a company of that size, this must be trivial for Shopify to implement. Have a search engine that can search across all their stores, let stores to opt-in, curate/categorize the products very well, let users search with very narrow parameters, and above all, don't do "sponsored" listings (charge a $10 fee per store per month, or something like that) . The only tricky thing here would be the order of listings - which product comes first among competing stores? That would require some thought.
Imagine a brand new, tiny, unknown seller. This feature alone might be worth it for the seller to sign up with Shopify.
Imagine a brand new, tiny, unknown seller. This feature alone might be worth it for the seller to sign up with Shopify.