I haven't hear about these problems at Walmart and would think the scale of stuff sold is comparable. I think the main problem, as others have mentioned, is that Amazon commingles products from various suppliers (so everything that comes in from a manufacturer or a true vetted wholesaler + 1000's of "flip stuff from china" garage operations get mixed up at Amazon warehouses before being sold).

Right. Walmart doesn't let someone drive up with a semi-load of what looks like bags of King Arthur bread flour, mark the pallets with a tag, and cut them a check when they stock the items from that pallet onto the shelf. That's basically what Amazon does.

Walmart is known for having a very tight control over their suppy chains. IMHO, mostly for cost control, but quality control and authenticity are useful side effects.