Bro I can do 1 to 1 on the spot too if they allow me to put sticker or anything on the device. But they only want with sealed box. So how to know if they buy from other seller and when spoil come to me and but new one and change.

You know I got how many customer bring fake power bank with my receipt come change. And another even funny buy three. 3 days later 3 also spoil but the power bank so many scratches and very obvious used very long already.

I know very well how to run a business and thanks for your advise . Let us do a math.
One power bank sold earn rm20 . One cost rm110 . If let's say we were to change a new unit to  spoil one for every 6 pcs you sell one spoil you only earn rm10 . That is if only 1 spoil and if no hanky panky funny customer who wants to take advantage on our warranty.
That is call business flaw ,  not call business loss. The loss that I must absorb is the postage to China for every power bank customer return. Not every powerbank they return and I'll have to give them new one.