I’ve heard of worse things to do with unwanted gift cards (like letting them sit around and expire).
So what will a gift card get you? According to this story (but ignore their incorrect math), a $25 Target gift card is worth 670 miles or 3.9 cents per mile.
Compare that to what it costs to buy miles directly from United: 3.5 cents per mile.
Converting a gift card to miles loses you about 6% of the face value of the gift card, similar or better than what you can expect if you were to sell your gift card to one of the secondary marketplaces directly.
Here is one aspect of this that doesn’t quite make sense; From the United website:
From Plastic Jungle’s website:
United/Plastic Jungle don’t require gift cards to be mailed in, but if you were to sell your Target gift card directly to Plastic Jungle, you would have to mail it in. Go figure.