My response as an individual who recently managed 5100 iPads across 44 schools is that this is not difficult.
In addition, I question that an organisation would not remove them from their systems before on selling, assuming that the person who on sold them is someone who should know what they are doing.
In essence, the situation should be as simple as one of the following:
If any device has a soft touch management approach from an MDM (meaning that it is simply enrolled to the MDM to provide minimal control, eg. connect to WiFi and no other restrictions), then the device should be able to be reset/restored to put it back into normal use.
If the organisation has used the MDM to place restrictions on the device such as preventing the iPad from reset/restored, then you could try putting it into Recovery mode.
However if the organisation has protected the device by using Apple's Device Enrollment program (DEP) in addition to using Apple Business or Apple School Manager (ABM / ASM), then any of the above steps will result in the device being automatically enrolled back into the organisation's MDM.
In this case the organisation should have removed it from DEP and ABM/ASM before on selling / donating the device.