If you bound the drone to your account and then returned the drone to Amazon for something like a "I don't like it" reason with out undoing the binding and Amazon subsequently sold/sent that drone to someone as a "returned drone" purchase then the new owner will be unable to use the drone because the binding is still active.
There was a thread here or on the DJI forum where such a 2nd buyer had received a bound drone.
I recollect contacting DJI to see if they could contact the original owner and check if the drone had been returned and if so ask if the original owner would undo the binding, was one of the suggested solutions.
If you look in the profile -> device management sections of the fly app and if it is still bound to your account you will likely see the drone listed in the "bound" section.
If you did return such a drone you should undo the binding. It is not necessary for the drone to be present.
I bound a
mini 2, to test binding, and was able to unbind it with the drone switched off. I assume the app lets DJI 'know' that the drone is no longer bound when the app next has www access. I do not have a second DJI account with which to test things from a second buyer's perspective.
You have nothing to lose by unbinding a returned for "I don't like it" etc. reasons. I do not see any risk, was risk could there be.
I WOULD NOT send anyone your login details etc., they DO NOT need them, you can undo the binding with the drone xyz miles away form you. You need not even reply to the email though I would have thought they (DJI) might have quoted the drone's serial number as an indication of authenticity.
In fact I think you should unbind it if it still shows as bound in your device management.
It should have been done in the first place, if you didn't undo the binding you sent a lemon back to Amazon.
Various comments in the thread I mentioned said Amazon are incurring significant loss because of bound drones being returned, if that is correct then one option for Amazon is to remove drones from their very generous returns policy,