Want to know why? Out of the 1184 Middleman transactions via PlayerUp of account purchases on OSRS.... over 783 of them had issues within 2 weeks. And within 1151 of them, they had issues within 6 months. That leaves 33 legitimate sales. And I'm going to be honest with you, I bet majority of those people never logged back in to complain and just accepted the loss.
Moral of the story, Runescape is the biggest scammer community out there. Account selling is basically guaranteed to be recovered. The only reason to purchase an account nowadays would be to buy a pure, ruin it, then chargeback your money because that guy is going to scam you 100%.
The only people to be trusted are super high feedback people who's lives rely on their sales via Runescape. And they NEED to be original owners of the accounts. And even then, I wouldn't trust them. Once they scamquit/leave the game, they'll most likely recover all the accounts too for a final payday.