Google Shopping is already a reality, in beta status but a reality, and any online store can register its products on Google Shopping to appear on users' searches.
Google Shopping is not a store itself as one might think. Google Shopping has certain similarity in design to Google Images, where you can search for a product, filter by price, store, if you want free shipping, etc.. You can view all products, compare products by price, and finally, click on the product of interest linked to end at the online store that sells the product itself. Following this, you do not buy it directly to Google but the online store offering the product. This last statement is valid only for Spain, as in the United States and other countries work slightly differently. Do not forget that Google Shopping in Spain is still in beta status.