When you are set to Walmart Only or Default and the item becomes available via a 3rd party merchant only (like ~Third Party Merchant Name~), you will see the supplier price = 0 and stock status = In Stock. This is because the 0 price is a "red flag" to sku grid to put your item out of stock. Sku grid will not reprice you based on a 0 supplier cost. The stock status still shows as In Stock, indicating that the item is available via a 3rd party merchant -- that's the true stock status. But it is out of stock per your settings.  This was a correct action for Sku Grid to take, given your settings and the current status of the item. If you are okay with allowing 3rd party merchants, then you will need to edit the entry in Sku Grid and set the drop down under the supplier url to All Sellers.  Do a Recheck ASAP on the item.  Then Force Reprice it.