The pedal, even in a fault condition, should not trip the mains.
The PSU 'should' have a diode fitted to stop reverse polarity.
It should also have a re-settable fuse in case of fault condition (ie shorting).
They probably used a very cheap centre +ve power supply instead of a 'proper' centre -ve PSU.
It is up to you how you deal with it.
Feedback being left is ok for ebay - if you are happy that the pedal was fine then they probably are just trying it on.
and yes, you should have sold it on BC lol