Hard to tell, since we don't know what the seller has tried in terms of buttons. If you could get it into recovery or download mode, it should be possible to reset it or reflash the firmware - in fact, in that case it might be possible to simply reboot.
Alternative to force reboot might be opening it up and disconnecting the battery. All without guarantee that it would work of course, but might be worth a try as it's not very expensive.
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