You used book-a-courier on trademe? For a one off, just turn an old courier bag inside out. Or if you don't have one you could say cut down a new rubbish bag and use some tape to seal it. As long as it's a strong bag of the appropriate dimensions based on what you booked, that's fine. Obviously it has to be big enough that you can put a label on it (or they can if you are just writing the booking number on).
I sell lots of stuff for electronic enthusiasts...
Hope you used anti static precautions when handling and have it in anti static packaging prior to whatever you are putting it in to courier.