If the URL of the eBay item is known, it would be fairly easy to check it every once in a while to see what the available quantity is. But such a program would probably best be independent of eBid, since eBay wouldn't like to have thousands of page accesses every hour from eBid.
Someone could make a utility that you run on your computer that checks your email for "your item sold" emails, and then checks the quantity on eBay. It would then have to interface eBid to change the quantity. As far as I know eBid doesn't have an API to allow utilities to edit listings, it would have access both sites through the browser interface. Without an API, each time eBay or eBid changes its user interface, the utility would have to be modified to work with it again.
edit: I was wrong. eBid has added an API somewhat recently. It's here:
https://ebid.3scale.net/ EBay has an API, but I'm not completely sure how available it is to something like this, as at least a developer account and/or an AppID is needed. They could potentially turn off access to service like this if it encourages competition.