Be careful when using multiple eBay gift cards for a single purchase. It seems somewhat random which cards are used to complete the transaction. It's not necessarily the last card which will have any remaining balance.
I recently purchased an item using a total of 4 gift cards. I expected the remaining balance to appear on the last card. Slight panic when a balance check of the card showed $0! A quick call to eBay support found that it was one of the other cards I'd used that had the remaining balance. Luckily I hadn't thrown it out!
Apparently: according to this old forum post on Paypal, refunds go back onto your gift card.
If your gift cards are physical in nature, it's probably also a good idea to keep it's details.