At OEM Experts, we are direct with many of the major brand name manufacturers that make the actual product, and even then we spot check the inventory.
We do this not to verify the authenticity of the product, but to be able to document our findings, if any, and reassure our customers with documentation needed to back it up. For example: We get a load in of 5,000 Altec Lansing Bluetooth speakers, or Lifeproof waterproof cases. We would buy a unit from a major retailer (like Walmart) via the retail store at full price for testing. Then, we weigh the product against each-other and break the item open to be able to compare the inner components and materials. We do this so that we can thoroughly document our findings.