Case in point if Marshalls gets a large number of returns for Widget X because of a defect, how can you be sure it came from the most recent shipment of widget x or a shipment that sate in a warehoue or store for 2 years? You cannot with any certainty - without your own manufacturing identification system.
Storage conditions are also important, but again once it has left the factory or distributor that is out of your control, no matter who purchases it. Granted, 3rd party items are more likely to have been stored incorrectly, but that is not exclusive.
As to returns, Amazon deals with returns all the time. If as a seller you fulfill from thier centers (IE any Prime sale), they will take the return. If a customer returns to the manufacturer instead of the store (marshalls, amazon, wherever), it is because they are making a warranty claim, and well you need to be equipped to handle that regardless of the source of the sale.