This engine uses the following search URL: https://www.ebay.de/sch/i.html?_nkw={searchTerms}&LH_Sold=1&LH_Complete=1
https://www.ebay.de/sch/i.html?_nkw={searchTerms}&LH_Sold=1&LH_Complete=1
To add it, you need to enable the separate search bar in Firefox. It will then offer an option Add “eBay.de verkauft”.
