There's a whole bunch of Cardpool threads on Slickdeals, it seems like this (and other gift card resale sites) is basically gambling.
There's two ways you get scammed on this, #1, they send you the card numbers, but they use the card themselves and it's empty by the time you use it. #2, people steal merchandise and return it for store credit and sell the store credit to these sites. When they get busted, HD/whoever will empty the card and you're left with zero.
I've thought about it myself for larger purchases, but in the end, I usually just end up skipping that chance because saving 13 bucks on every $100 isn't worth potentially losing 100 on every 100. I'll take my 2% cash back and extra warranty through my credit card.
The common advice is to use the cards immediately after you purchase them, but it's still potentially taking part in criminal activity.