Great, I recently worked with a non profit who sells Pashmina shawls from kashmir on ebay and also to European suppliers (wholesale). Luckily they were using magento kind of platform that offers multiple customer account types (for wholesalers and retailers).

This is an integration question. How do you integrate your ebay selling event to your inventory (inhouse / cloud) systems.

Option 1 - Hire a freelance and build a hook using EBAY api - http://developer.ebay.com/devzone/guides/ebayfeatures/Notifications/Notif-ItemSold.html
OR 'Out of stock API
http://developer.ebay.com/devzone/guides/ebayfeatures/Development/Listings-UseOutOfStock.html

Option 2 - Wait for Zapier web hooks. It is in their wish list.
Option 3 - Some scraping hacks :-) Not the best way but definitely a TACTICAL way to see if th product is in stock on not...

Option 1 is the neatest one and scalable.
Are you using a online/saas product for your wholesale business.. Just curious ...

If you need any help in implementations/online-presence strategies..let me know. Happy to speak !!