The resale value is the only real drawback. That said, even the right engraving could make things okay. Engraving "for my sweetheart" is pretty generic and could still sell. Engraving your home address or someone's full name... not so much.
It depends on who you're getting the iPod nano for and how attached that person gets to special items. If they're the kind that would eBay their own mother, don't get an engraving.
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