If a seller uses the Sell Item form without assigning the Collectible he or she is selling to a Database Item than it is a "Generic Item" in our system.  If the seller than cancels the sale or if a buyer buys this item it loses its title (this is something we plan to change) and is titled Generic Item and shows in your Showcase like this (the Estimated Value is for all Generic items and thus not meaningful)  -

You can easily find these items by using the Generic Items filter either in your Collection Management page or your Items for Sale page - 

You can either mark the item as private or if the right Database Item has already been added to hobbyDB reassign it in Collection Management  -