Tefillin of unknown craftsmanship found in the possession of an apikorus should be put into geniza. (ShA 5)
We are obligated to buy tefillin, mezuzos, and seforim from the non-jews possessing them so that those items do not become desecrated. (MB17)
You may only buy tefillin from a mumcheh. An established public scribe is a mumcheh. (ShA8 – MB19)
Mezuzos and parshios for tefillin may be bought even from a non-mumcheh on the condition that you have them inspected. (MB19)
Tefillin which have a chazakah to be kasher never need inspection. Nevertheless, it’s proper to inspect them because sweat can ruin them. But tefillin that are worn only on occasion require inspection twice every seven years. The Chayay Odam says you may say a beracha on them until you find someone to inspect them.(ShA 10 – MB 26,27)
If tefillin become soaked, they must be inspected immediately. If you cannot get another set before the tefillin are checked lay them without a beracha. (MB26)