Amazon's gift code algorithm isn't exactly rocket science. IIRC the code generators work for 4-8 months then Amazon catches up and shuts them down. IIRC Amazon throws a pretty wide net out to try and recover funds from those who used illegitimate codes.
You should never rely on an emailed code. If you don't visibly see a code on a plastic gift card there's a good chance it's illegitimate.
This is what a legitimate seller looks like:
I'd love to see some proof/evidence that any of these guys are selling legitimate codes.