I don't know about that specific site, but I have done it and it works. I thought $16 was the resell price. I've talked to pokerstars
support and they say its allowed you just can't discuss trades at their tables.
How its done: buyer is sitting at a playmoney table, you click on them and transfer the playmoney after talking to them on a messenger service. Once they get it they send you cash. Sure you have to trust them, but their name would be trashed if they didn't follow through. What do you have to lose anyway, its just playmoney.