As someone that has used these services before, it's unlikely that it's a scam. Gold farming is a huge business for some people in Asia and some gold sellers have tens to hundreds of people working with them, so it's hard to say which is a scam and which isn't.
This is what these people do for a living, I'm sure they've devised methods of optimizing game time and gold gain as a team.
edit: There HAS been times where the gold farmers will be "out of stock" but I've always been compensated for the delay in receiving my purchased goods. But again, this is all my experience and it doesn't account for everyone's experience or the legitimacy of all gold farmers.