Can the Apple gift card be used at resellers?

No.

As far as I know, no. It can only be used at the Apple Store or the online Apple Store. Quote below from Apple website.

"Use your Apple Store Gift Card for the purchase of a new iPad, iPhone, Mac, or anything else at the Apple Online Store or Apple Retail Store."

Clearly you don't understand that gift cards are a form of lock in.

I believe you can bring your Apple Store gift card into one of their retail stores and buy an iTunes gift card.

Note that you can buy $100 iTunes gift cards from Best Buy today (Black Friday) for $85.

trans-olsx said:

What about to buy an iTunes Gift Card of the same amount?

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Only if you purchase said gift card from Apple, either in store or online.

trans-olsx said:

such as for applecare, etc.

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I've been assured by employees in the Apple Store, that those gift cards can indeed be used to purchase AppleCare. Not sure about resellers though.