Earlier quoted context omitted.
Not exactly accurate. There is eBay that is a big player and allows anyone to distribute products. These days 90% of sales are done by professional in fix price format (not an auction). eBay has a proper reputation system, sellers are clearly identifiable and so is the location of the product. Amazon doesn't do any of that on purpose.
Well sure, but AFAIK eBay doesn't do fulfillment so there's no possible issue of commingling, etc.
Or well, sometimes they do, sometimes they don't. It's impossible to figure out where a product will come from.