Need autorisation from Microsoft to sell my used consoles on Amazon.

Since we are not the team who deals with such requests, it would be best to reach the sales department. You can be directed to them by getting in touch with the online team. Access Contact us | Xbox Support. Follow the instructions after clicking on the green button "Contact us" and choose to get help either from an online chat agent or request a callback. Make sure the region on the support website is set to your correct region in order not to find issues with the steps.

Microsoft needs retail to sell the consoles.

If Microsoft sells digital downloads for less than the likes of GAME or Amazon then those stores could retaliate by refusing to sell the consoles.

Microsoft are starting to sell their console from the MS store, but for this generation that is just a fraction of the total consoles sold.

And when we all say "brick and mortar" stores, we basically mean anything other than Microsoft. I mean can you imagine Microsoft undercutting Amazon on digital games? Amazon would tell Microsoft where to go. Same with Sony and Nintendo, they all need retailers
to sell their goods.

Basically, Microsoft (and Sony for that matter) are only charging the RRP on games, most games you buy from Amazon etc are actually sold below the RRP because they buy in bulk and get discount.

Also can you really see Microsoft/Sony selling digital games cheaper than Amazon or GAME etc? what do you think they will do? Microsoft needs Amazon and GAME etc to sell the consoles, so why would you undercut the people who sell your hardware?

If you need a new power supply then unfortunately Microsoft no longer sell them, but you can buy them from online sites such as Amazon or Ebay.