[gamblor956]

The distinction comes down to the legal differences in how they operate their sales operations:

Costco is legally a "reseller" that purchases items from manufacturers/distributors (at wholesale prices) and "resells" them to customers. (Note: only a tiny fraction (<1%) of Costco's inventory is sold on a consignment basis, meaning that the manufacturer/distributor only gets paid for units actually sold on. This arrangement generally only applies to some new products being sold on a trial basis.)

Amazon is also a reseller of items sold through the Amazon.com seller...but not for items "sold" by third-party merchants. The distinction is that for Amazon.com seller sales, Amazon has legally purchased the inventory sold, even if the payment terms may more closely resemble consignment transactions than wholesale transactions.