Except for the private feature you mentioned, the Collection Manager can be used for this. th30d even used this idea and created a simple one called "WANTED
". Descriptions could be added to the top of the page ("These are the puzzles I'm searching for.") and to each item ("This one is for sale in the shop on this site."
). Items that aren't in the museum can still appear on your collection page, but will appear without a picture.