If you need to change stock/inventory levels please do this out of Shopify/BigCommerce/WooCommerce. eBay LINK keeps in sync with the stock/inventory levels there and uses this information to update eBay.
Shopify and BigCommerce inventory is updated in eBay LINK then eBay in almost 'real time'. Each time inventory changes, eBay LINK receives the change and then updates eBay.
For WooCommerce, inventory is updated when there is a full sync between WooCommerce, eBay LINK and eBay. This is once per day and usually scheduled for overnight. If at anytime you want to update the inventory outside of the automated sync, change the inventory in WooCommerce, login to eBay LINK and from the Dashboard click 'Import from WooCommerce'.