Buying a Costco membership doesn't mean you can only shop at Costco though. If Wal-Mart has a better price on X or if Mom and Pop shop has item Y that Costco doesn't carry you can still buy them.
And buying an iPhone doesn't mean you can't also buy an Android phone. The analogy you're looking for is that Costco does not indiscriminately allow all sellers to sell products at a Costco warehouse.
