FWIW, you wouldn’t be able to do it entirely in Unbounce. eComm gets pretty complex, and out-of-the-box solutions are your friend. (Noting you don’t already know .) You’d still need a payment gateway, order management, etc., and cobbling that together would suck. The Shopifys of the world are built for that – turnkey.
Now, Unbounce can be a tremendous front end for eCommerce, particularly on the CRO side. I’d much rather point my ads to an Unbounce landing page with Smart Traffic turned on than a basic Shopify page. But I wouldn’t build an entire store there, even if you could. Wouldn’t be expandable in any way.