I guess they must have an inside supplier or something to get them so cheap. But anyway even if it didn't work ebay is safe nowadays so if one didn't work and the seller didn't contact you to resolve it, paypal will give you a chargeback (refund).
100% rating means nothing if the account has been hijacked. Which happens often enough. And if you use gift cards that have been purchased using stolen credit cards, you'll have a problem. Like your iTunes account might get closed.
You can always find some store selling iTunes gift cards with 20% rebate. If Boots or Argos sells them, you know it's genuine.