Where do you see, that the item is out of stock on eBay? Do you use the Selling Manager? Maybe a simple screenshot would help here.
Why do you think GarageSale thinks that there are items available? Do you use the inventory to track your quantity left or do you just look at the quantity field in the listing?
We do not update the quantity field in the listing. An inventory item is the way GarageSal tracks the quantity left. You have to create an inventory item and connect it via the SKU field with an item. Whenever an order for the listing is created, the quantity in the inventory should be decremented.