Hi Heeseong,
Do not sell a preloaded Windows USB and do not resell product keys you found online. Both run against Microsoft's licensing rules.
Windows is licensed, not sold. The Windows 11 license terms prohibit publishing or copying the software other than a single backup copy, and they restrict transfers to very specific scenarios. Shipping new USB sticks that already contain the Windows installer for resale is a redistribution of Microsoft software that those terms do not allow unless you are distributing official Microsoft media or acting under an authorized program.
Microsoft's own download page makes this even clearer. The Media Creation Tool is provided so end users can reinstall or clean install, but it explicitly says you first need a valid license. In other words, the tool and media are not a product you can package and sell as a shortcut.
If your business goal is to preinstall Windows for customers, the compliant route is to become a System Builder or otherwise work through Microsoft's authorized channels (More information).> System Builders must follow the OEM System Builder License and sell Windows preinstalled on a device, not as a loose installer.
Regarding keys sold on websites, most of the cheap keys circulating online come from channels that are not permitted for resale, for example Visual Studio subscription keys or other non-retail sources.> Those keys can activate for a while but are not valid licenses for production use and can later be blocked. Microsoft's guidance is to purchase Windows only from Microsoft or authorized resellers and to use Genuine Windows programs if you need to correct licensing.
If you want to help customers save time in a compliant way, sell a blank USB and your service time. Sit with the customer and use Microsoft's official Download Windows 11 page to create media on their behalf, and have them buy a genuine Windows license from Microsoft or an authorized retailer. That keeps you aligned with Microsoft's terms and avoids the risk of non-genuine licensing.
I hope this helps!