Well, the macrumors forums are not a court of law, and when last I look, it was still up to a court of law to decide whether something is legal or not.
That being said: Those guys sell used Macs and machine parts, like your next door scrap dealer does with cars and car parts. That can hardly be illegal in itself and you certainly don't need anybody's permission to do so.
Their prices are also not outrageously low. And even if they were - enough companies file for bankruptcy and have to dump their equipment for peanuts, so there's a perfectly solid explanation for cheap, yet good gear - it mustn't be stolen or otherwise illegal. Just find somebody who is in desperate need of money and you'll get your bargain deal.
When you think their prices are okay and they actually ship you the machine that you've purchased in the described condition, what's wrong with that?