I have seen Starbucks at like 35% face value. What are the repercussions if you order stuff to your home address using them and getting arrested? You can not prove you have not stolen them?
Starbucks cards are likely to sell at a much bigger discount then amazon cards because you can buy far fewer things at starbucks then you can at amazon (you won't buy a starbucks gift card for consumption unless you like to drink starbucks coffee while you can buy almost anything with an amazon gift card), and there are legit reasons why someone would be willing to sell gift cards at a discount (won on a contest, received as a gift, received via reward points, ect). As a result the starbucks sales may have been legit
If you do use a gift card that was purchased with a stolen credit card then you run the risk of being implicated in credit card fraud. The chances of this increases as you deal with larger amounts of value of cards. IMO you should generally stay away from dealing in the sized value of gift cards the OP is attempting to purchase
That's why you don't use carded GCs on your own Amazon account. Ones aquired through other means you have little to worry about. They have no way to prove you knew the gift card was obtained through fraudulent means anyway. The most that will happen is your account will get shut down and if you have a CC on file they will try to charge it. That's actually in their ToS. Might try to blacklist you also by banning your IP but there are a multitude of ways to get around both those things.