To be an authorized Apple Dealer, a company must commit to providing additional services, and must buy a large amount (like US$250,000) a quarter of Apple stuff.
A company can choose to be an authorized Reseller for a much smaller comitment, but then there needs to be some unique value added (such as bundling your custom software) to qualify as a Reseller.
It is possible costco is trying to get in as a Reseller. In the US, Resellers try to quailify by bundling third-party Utilites or private-label warranties with the Macs they sell.
If they are not meeting the requirements of either program, Apple will eventually catch up with them and try to enforce their rules or cut off their supply of Apple stuff.