What are the actual risks regarding buying these codes? Sure, I know the seller can keep the code after you sent you BTC, or he can send you a invalid code. However, I do also read replies over here that people are that these codes can be bought with hacked accounts and thus stolen money. Is that true?
Will it harm me if I use such "illegal codes"?
Amazon gift cards are one of the more notorious scams on here, especially from new accounts. Main issue is that they're purchased with stolen credit cards, and once the owner files a dispute, the gift card will get voided, even if you already redeemed it to your Amazon account. If you redeem it and quickly purchase something before it can be reversed, then you may be at risk of having police knocking on your door.
I was scammed for ~$800 in amazon gift cards in 2011 when they got reversed. There was a seller dumping mass amounts of these for a discount. I believe this was the first Amazon GC scam on this forum, so no one knew of it yet.
Just be careful with who you deal with. Just think about it, why would anyone take a hit by selling it at a large discount if it's a legit GC?