It's like buying recharge cards. You can only check if the codes are correct when you adding it to your account.
Is there any way to protect yourself against hoodlums ?
You can protect yourself by buying physical cards from well-known stores, the percentage of codes being wrong with them is minimal and you can stipulate that the buyer send money once he gets the code and there is no refund service.
Some P2P sites provide excellent service for buying and selling such cards.
Can I send money from my account (which I have from the code) to another user's address even if they have an account in another region? For example from Amazon UK to Amazon USA ?
You can’t transfer funds between your Amazon marketplaces directly, but you can enter bank details (obtain your IBAN from) on each marketplace.
There are some third-party apps that enable you to do this but they are not guaranteed and may be scam apps.