I would add a boolean type field as "SOLD" to reduce the first Condition.
PS: Drop down lists are over used, in my humble opinion. Portals in FMP11+ are much more powerful.
Dwindling Value Lists are conditional value lists. They just include additional details in the relationship so that, as Jim mentioned, changes in a field can then omit that record's value from the value list. If you had a text field that stored the value "Sold" when it is sold, you might change the relationship used for your CVL to be:
//keep your original match fields here
AND ParentTable::constSOlD ≠ ValueTable::Status
ConstSOLD would be a calculation field that returns the text "SOLD". And you don't actually have to use the text "SOLD". A number field can be set to 1 to mark the status and then you can use a calculation field, constOne to match to it.