Sku Grid updates your listings based on supplier changes, ie. supplier price and stock changes, shipping cost changes, etc. Sku Grid pushes out information to your marketplace, but does not receive information from the marketplace. When your items sell on eBay, because this is not a supplier change, Sku Grid does not receive any indication that it needs to be updated. 

eCom Syndicator does not natively update after sales either. Typically, after a sale, you would need to click on "Force Reprice" for that item, while fulfilling the order. This would update the quantity on eBay to match your default. Now, Sku Grid offers Orders Dash! You can set up Orders Dash to auto-update your quantity after sales. 

Click the link below for a full walk-through/tutorial on how to get started with Orders Dash!

Getting Started With Orders Dash