If someone who borrows my tefillin marker uses it without saying “l’sheim kedushas tefillin”, would that invalidate the marker for further usage?
Answer:
The marker remains fine, and can be used in the future for painting tefillin.
The tefillin that were painted should be painted with another coat of marker-paint, and this time with the prior declaration of “leshem kedushas tefillin.”
Best wishes.