Eg. 12.872btc selling for 6btc
The reason they are willing to lose money is probably because the bitcoins are stolen.
That would be the most expensive mixer service ever! Or, more likely, a scam: you lose your 6
BTC, and never get the 12.872
BTC.
Even if it would be true, and the seller is trying to make a tracker lose the scent of his 12.872
BTC, the seller can't know if the 6
BTC he gets aren't stolen as well. One way or another, it makes no sense selling it at this price.
It is true, I think it is simply a fraud and they want to capitalize on something that you don't know. Better never tempted by something that does not yet know with certainty, because if you're wrong step it will make you a bad person