No, it is
not illegal to sell an OEM license. Per MS licensing agreement, you can transfer an OEM license, as long as you are also transferring the hardware that it it installed on.
However, that was not what was asked.
Is it possible to sell your Windows 10 ISO?
IF someone had purchased Windows 10, of course you can sell that license. You obviously can't se that license anymore, but you can sell it.
In reference to the following statement:
Not everyone out there has genuine windows in order to upgrade. I'm asking like if one has upgraded to Windows 10 and has an ISO with his own license, could he sell it to other people using that license key that he has or not?
This doesn't quite make sense. If you have a valid Windows 7 or 8.1, an upgrade to Windows 10...no, you cannot sell that Windows 7 or 8.1 license. You have used it up in the Upgrade to Windows 10.
You can't Upgrade it AND sell it to someone else.