For online sales, you can associate the product variant to a warehouse if this product is shipped from a different location than your business address. After checkout, the variant's inventory will automatically be reduced. You must first enable the Warehouse feature under Configuration > General to use warehouses. Please see Warehouses for more information. 

If the same product is available from different warehouses, you need to setup one variant for each warehouse so that the inventory is correctly tracked back to that warehouse. Rather than letting the user decide the variant to buy, you may want to configure the variant availability rule to show a particular variant from a certain warehouse depending on the user location. For example, you may want to use the Availability rule to detect if the user is visiting your site from California and present only the variant that is stored in your West coast warehouse, and do the reverse action for a user located in the East coast. Depending on the use case, you may need to employ 3rd party detection software like MaxMind GeoIP lookup to cookify the user visiting your site so that your Availability rule can correctly determine the precise location of the user.