Um... only because you believe that nobody will do that doesnt mean that youre right. In fact purse.io is doing it since a longer time already and they collected, i believe, 400K USD in seed funds. Check out the support thread on bitcointalk about it. You can get 50% discounts. I did.
Unfortunately they become more and more restrictive. Didnt allow buying of amazon coupons anymore, then stopped external products in wishlists and now a whole country is blocked. I wonder if they fix their things.
You'll probably get the items just fine, but you never know how the item with your name on it is paid for. Do you think somebody that legitimately earned those cards would give them at half off? You're giving anonymous currency for something sent to your house...
They stay in business since a longer time now. So it seems to me the problems you see arent happening often. At least i would await users writing about that in the support thread.
And as far as i know purse.io is aggressively advertising the possibility to exchange ones amazon coupons into bitcoins. Im not sure where people get all these coupons from but there are a lot of people who have many of them.