There is a bug with keybinds that stops you from trading credits to anyone. If you bind certain things to 0 it can stop you from trading credit amounts ending in 0 to any players, I've had this bug multiple times over the years and always had to delete my prefs to fix it, I just got it again on an alt that can't trade and decided to find what part of prefs.xml was causing the problem. On my current alt if I bind "Select Team Member 2" to 0 it causes this bug, but I am pretty sure there are other binds that when set to 0 will do the same. I can reproduce this bug easily and send a prefs file to the devs with the bug if they want. The bug may be dependant on having other stuff bound to 0 also, I have 0 bound to stuff on other chars without the bug, but on my doc I can 100% reproduce it with it's current bind order. Maybe it happens with whatever number you bind to certain things then you can't trade cred amounts with that number in it or ending in it?
If you have this bug and want to fix it without deleting your whole prefs folder you can just unbind 0 from stuff till trade works, or go into prefs.xml and delete the keybinds section and set them again.
The section starts with <Archive name="KeyBindings" code="0">
Hopefully if someone searches the forums for this bug they can find this post in the future.