[WorldMaker]

Microsoft guidelines appear to state that given the Ship of Theseus nature of machines there is a two-factor of sorts authentication for the license of that machine: the license code sticker, and the original recovery disc (as a harder to counterfeit physical token than the exact state of hardware of a machine; remember all the XP era relicensing woes when you swapped out a RAM chip?).

If you sell all three together on eBay, that's entirely fine, and it doesn't need a relicense fee. It's when you start to lose one of those three that Microsoft asks you pay a relicensing fee.