Does the seller know the password, weird it's in airplane mode, he probably stole it and enabled Airplane mode so it wouldn't connect incase owner remotely erased it.
What is the cost of this piece of junk that looks as if its only good for parts ? Don't be so gullible OP. Not worth it. Save more money, than buy from a better source
If you have your phone less than a year you may go to the service center but if exceeds the warranty period, you may need to save enough because we never know the prices of different sellers.
You need to raise your budget/price range or don't bother at all.
This is most likely a broken and useless device with icloud lock.
Stick with a flip phone.