Click on the Wines tab
On the left-side menu, under Warehouses, click on Manage
Click on the desired warehouse
Click Edit
Check Marketplace?
Click Update
The red "M" icon indicates it's available in the Marketplace
Please note: When selecting multiple warehouses for the Marketplace, the Inventory Status (i.e. In Stock/Available, Limited Stock & Please Inquire) will show the total combination of all inventory at all warehouses.