You can actually create your own unique and copyright product if you use public domain products in a certain way.
It's
called "derivative work" and it is a method of recreating
existing public domain content by incorporating your own material as
well as existing material so that it's a unique collection, book or
piece.
When
you do this, the portion of the material that you created is
instantly protected, while the portion of content you extracted from
the public domain continues to be available for use.
This
is a very smart method of creating fresh, new products by blending
existing content with your own unique material, so that you can
present it as a new creation, revised or improved product, and one
that cannot be directly replicated as your portion of the work
belongs exclusively to you.
One
thing to keep in mind as well, is that if you wish to prevent someone
from taking an exact copy of material that you have pulled from the
public domain and are selling online, is by following the “minimal
creativity” requirement,
which means that you must add a substantial amount of original
content to the works, in order to claim a valid copyright.
Minimal
Creativity requirement prevents people from pulling material from the
public domain, ‘as is’, and then claiming copyright to the work.
