1. UM site prob wouldnt let you do that. as a matter of fact, it'd be dumb to let anyone buy a giftcard with a giftcard since there's no good reason to in the first place. Now that they're doing this giftcard sale, they will definitely not let you do that.
2. in the case you succeed doing so, your gift card will probably get cancelled. (the one you bought with a giftcard)
3. its a good way to get blacklisted from the shop since youre exploiting something that shouldnt be