Where I live onselling is illegal, but it is still a pretty common practice for small online retailers to just not stock some items and go buy them at retail when it's ordered. It makes a little bit of sense, since it lets you expand your catalogue without keeping stock on hand or negotiating a supplier. For a small ecomm store, that can mean the difference between looking like a legitimate retailer a customer can trust or not.

This is different to the practice of buying up stock on sale and reselling on marketplaces.