I highly doubt it's a scam. The site he's linking offers similar service for a ton of other games, something multiple sites have been built around. Site even has a disclaimer your account could be terminated.
But, all he's advertising is basically 50% off what Sony/PD's MTX pricing is at ($5/1 million credits). Prior to the update & PD acknowledging the upset fanbase, this was probably a really enticing deal. I just saw a member say if the MTX was a better value, they'd give Sony $40; this is basically what that member probably wants for the value.