From Webkinz Insider Wiki
Retired items are items that no longer sold in W-shop or Curio Shop, but may still be available from other Webkinz World members for trade. They are sometimes very high in value. A lot of retired items have the word "valued" in their title in trading.
Pictures of retired items can be found in the Retired Section of the Wiki Picture Gallery.
The value and date of retired items can be found in the Retired Section of the Webkinz Insider Gift Exchange Value Guide.
Many retired W-shop items were first shifted to the Curio Shop before retirement. Some of the history of when items or whole themes were shifted and retired can be found in the summary of Significant Events in Webkinz World.