Is one allowed to sell tefillin to someone who is GOING to convert to judaism (therefore, learning to put on tefillin) but hasn’t yet actually converted?
Yes, this is fine.
As we have explained in a previous post, the prohibition of selling Tefillin and similar items to non-Jews is out of concern for disrespect to the item.
This does not apply in this case, and therefore there is no problem.
Please see here concerning the different yet similar question of giving a mezuzah to a non-Jew.