That gift card was tampered with on the store rack, before you purchased it. Scammers scratched the numbers off the card, recorded the numbers, damaged the numbers so you would not be able to use the card, put new scratch-off material to make it look untouched, and put it back on the rack. Then they waited until the card was purchased and activated. They monitor the cards they have tampered with, and very likely they had spent the money before you got the cards home.
I don't believe eBay can help with this, because they won't take responsibility for poor security at the store where you bought them. You can try contacting the store, but they may or may not be willing to accept responsibility. You may need to contact the credit card you used for the purchase and do a chargeback.
Unfortunately, the only thing you can do to avoid this, is to not buy cards that are accessible to customers in the store before they are sold. Only purchase from stores that keep their gift cards away from buyers, behind security barriers. Or don't use eBay gift cards, that is safest.
