You can add an ebay auction to the game's page by doing the following:
1. Go to the game's page on BGG
2. Look for the ebay module, and then click Add within the module.
3. If you know the ebay auction number, click Enter auctions by ID and then enter it. If not, put terms into the search box and hit Go; then find your auction in the list and checkmark it.
I am not finding a EBAY module....