This afternoon an idea suddenly came into my mind. Which gift card would give you best resale value? Since we have a lot of Membership rewards points from AMEX, Thankyou points from Citi and world points from BOA, we might use those points to redeem some store cards. Then here comes a problem. Which card will give you best value? Believe me, different gift cards will have different real values. (I suppose you want to sell the gift cards at ebay for cash.)
Then I did a little search at ebay. I took 5 recently completed auctions for each popular gift card, like Home Depot, Target, Best Buy, Staples, Amazon, GAP etc. I then counted in about 5% eBay auction fees and then calculated the net resale percentage (shipping was not counted) according to the following equation:

Ending Bid x 95%) / Face Value = Estimated Resale %
Even with only 5 data points per store, it will give us a hint.
We all might have known that Amazon.com gift card has the highest resale value. Because you can easily redeem it online, and they essentially sell everything from groceries to electronics. Target and Home Depot followed closely behind. The worst performing out of this limited sampling was Gap. Banana Republic, and Old Navy also hovered about the 80% range.
So if you want to redeem some gift cards, you will know which one will give you best value.
Of course, this post also gives you a hint: use store gift card to save money! For example, you can only get 2% cash back when you shopping at Staples. However, if you use gift cards, you can save as high as 10%. If you use credit card as PayPal payment, you can still earn cash back from credit card!