not sure he is a scammer; I doubt Coindesk would advertise a scammer would they?
Do you think they do due diligence on everyone who has a quote in an article? I seriously doubt it. This guy is a total scam = but do your own research before trading, of course.
All that coindesk is doing is reporting an offer that someone has publicly given. They are not endorsing the person nor saying that he will not scam.
If he were to try to buy as much bitcoin as he is trying to buy on an exchange he would likely not see 8% slippage so it would make much more sense for him to simply buy on an exchange.
The only potential way this would not be a scam is if he is trying to buy up a lot of bitcoin but also wants privacy. I do think he can buy up a lot of bitcoin while still having privacy without offing such a large markup, but he may want to buy his position sooner rather then later.