According to this Auctionbytes article, Yahoo! has elimated ALL fees on their auctions - both listing fees and commissions.

Rob Solomon, Vice President and General Manager of Yahoo Shopping, said on Monday that Yahoo monetizes the auction platform with its core graphical media (banner advertising) and search-based services.

When asked about concerns whether free listings would actually decrease the quality of the listings since anyone could list an unlimited amount of items, Solomon said Yahoo Auctions is utilizing anti-spam technologies developed for all Yahoo properties, including Yahoo Mail and Yahoo Groups, and technologies that detect fraud.

I predict Google will show up in this space sooner or later, and with this exact model (since it is, after all, the model they apparently use for all their services).