back.? Let's say one's got a credit card and bought something expensive on Ebay, and later on he doesn't have the money to pay it back. And the item he bought was a gift, and it has already shipped to someone(whoever). Does the one who got the gift have to return it? Since the buyer doesn't have the money to pay the debt to the credit card company?
And what if the item bought was a gift card? Does the credit card company has the authority to make the gift card go invalid?

No, credit card companies don't typically re-posses items when payments are not made on a credit card. What happens instead is that the credit card will be in default - which means you have not made payments and the balance is over due. The card will charge off and then you're sent into collections. Then the bill collectors will call you to work out a payment arrangement. If you still do not pay off the debt then it's reported to all three major credit beaurus. Worse things can happen too like freezing your bank accounts if you have any and making it difficult for you to get other types of credit.

If you don't have the money to pay back the credit card company you will simply default with them which will affect your credit rating. Technically, they can attempt to recoop goods bought with a credit card but normally that applies only if the buyer is still in possession of them i.e. they can't prove it was a gift and that you have shipped it to someone else, as you could have just sold it on and pocketed the money. If it's a gift card, then it's possible they could appeal to the company who issued it but it's unlikely that the issuing company would go ahead and invalidate the gift card as it's not good business for them to do it. But it's a slippery slope my friend as if you don't pay, your credit rating is toast. Better to contact your credit card company and make a payment arrangement with them, which allows you to make smaller payments each month till the item is paid off.

thats between you and the credit card company. not the seller. now if the seller has a return policy, you probably can get your money back unless you return the item.