Thank you for the link - Section 3.8 seems the most applicable
"3.8 … You agree that you will not, and will not allow any third party to,
(i) copy, sell, license, distribute, transfer, modify, adapt, translate, prepare derivative works from, decompile, reverse engineer, disassemble or otherwise attempt to derive source code from the Unity Asset Store or the Assets, unless otherwise permitted
So if I received permission from the author of N-GUI it may be permitted to transfer the license to another party. Then there would be issues of them getting future updates blah blah blah