regarding Eid Echad Neeman, there is a takana of the Vaad HaArbah Aratzos not to BUY from anyone who does not have a Hechsher/Haskama from a recognized Rav/Av Beis Din
I wish i would remember the exact Mare Makom, I saw it somewhere in Pischei Teshuvos.
I have no idea if this takana is relevant to Lubavitchers, or if they have a different mehalach