If you wish to leave a seller feedback then go to your Purchase History page far right from More Actions is 'Leave Feedback'.
Be aware, if you made a Guest Purchase then there's no facility to leave feedback. You also have 60 days in which to do this, after that time lapses, it's not possible.
There's also recently been temporary tech issues with feedback so it may just be case of .... please try again later.
@nja-noe-dfn6snxxv
