Bad customer service

I bought a charger from them and within a week of using the charger I realised that it wasn’t working and instead depleting the battery. I then sent it back for a refund and they said they can’t due to the fact it has been opened. Please explain to me how was I supposed to know the charger doesn’t work if I hadn’t opened it???

They didn’t even try to exchange it but brought back the same charger and I am sitting here with a charger I paid for and it doesn’t work???

Was I supposed to determine it doesn’t work by looking at the box. Would that have made sense??