|OEM activation is done locally based on the hardware it is running on. The manufacture know exactly what hardware the OS will be running on because they made both the hardware and the installation disk.|
If you sell a computer with an OEM installation on it, the installation and the licence and any disks should go with it. Selling an OEM computer does not invalidate the activation.
I don't understand what you mean by typing in a User name. Users names have nothing to do with activation and there is no limit to how many users can use a single installation.