These are miles sold off by people that don't need them or have an excess, and then they're collected and resold. It is of course against the Ts + Cs of the carriers for anyone to either buy or sell miles or any other perk given in loyalty programs and they're not transferable.
It's entirely possible to get away with it, but it's getting far riskier. Most decent airlines employ fraud and revenue protection teams to actively search for this activity, and they'll cancel the ticket and sometimes ban the passenger/s concerned. The fraud detection success rate used to be very low (in one carrier it was less than 20% 3 years ago, now it's over 80% thanks to investment in tools and algorithms which can spot the activity). I'd never risk my ticket on these schemes, I know that much.