It seems the plume feathers that are not used in the minifig are extra parts.
The extra plumes are seldom found with used sets found at sales. Keys in the
Harry Potter set are also confusing in this regard. Other parts like 1x1 round
plates come together and the unused ones are considered extra parts. It does
not seem right that a castle must be listed as incomplete if an unused plume
is not included. This is especially true of black plumes.
In Inventories, Zab3uk75 writes:
Hi, can any one confirm the head gear plumes in this set.
The inventory (Bricklink) shows
Plume feather small 4502a x 1
plume feather large 4502c x 1
The instructions show neither of these (perhaps I have missed them) but does
show
plume feather triple 4502b
It seems the plume feathers that are not used in the minifig are extra parts.
The extra plumes are seldom found with used sets found at sales. Keys in the
Harry Potter set are also confusing in this regard. Other parts like 1x1 round
plates come together and the unused ones are considered extra parts. It does
not seem right that a castle must be listed as incomplete if an unused plume
is not included.
It can be listed as complete per definition of used complete set:
Complete - Set contains all parts necessary to build all of the models
(including alternate models) in the instructions but does not have to contain
the original box/packaging and/or extra parts. If the set is missing instructions,
the description must list this. If the set did not come with instructions then
the set should include all parts listed in the picture inventory which is located
on the box or bucket. The set must include all original stickers used on the
model either still on the sticker sheet or applied.
This definition does not say "Set contains all parts in the regular
or alternate section".
If the plumes in a castle set are not used in any alternate model, the set can
be listed complete without them.
This is especially true of black plumes.
In Inventories, Zab3uk75 writes:
Hi, can any one confirm the head gear plumes in this set.
The inventory (Bricklink) shows
Plume feather small 4502a x 1
plume feather large 4502c x 1
The instructions show neither of these (perhaps I have missed them) but does
show
plume feather triple 4502b