How do I fix this
Hi @Shokarina, sorry to hear about this. 
I'm not quite sure what you mean with "Used for sale", but you can try and connect the device to a charger and wait a few hours and see if it repairs itself.
If that doesn't help, you will need to perform a software repair using Xperia Companion to reinstall the software in the device. Do note that this will delete all data stored on the internal storage.
Let me know if that helps! 
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