In addition to all of the above comments, we always insist that the seller (whoever that may be) includes a non-incidence statement with their shipping documentation so that you know you're not getting a unit which has been subjected to stress, heat or abnormal pressure. If in doubt about the trace or source, leave it alone.
Would recommend reading up on this document:
http://www.aviationsuppliers.org/ASA..._AC_00-56A.pdf
The FAA have a good basis (albeit voluntary) to address how distributors without an FAA repair station approval handle aeronautical parts. EASA keep on stating that they'll do the same thing one day, but nothing has happened yet.