This is a setting in specific listings that a seller can opt to use, so you won't be able to do it for any items already sold. When you list something, you need to select the box for "Private listing - Allow buyers to remain anonymous to other eBay users", buyers are anonymous to other users anyway, but it means the item title won't show up in the seller's feedback.
Just be aware that some buyers will be wary or suspicious of private auction listings, as they're often used to try and cloak things like shill bidding.
