wdmenke, I understand your point of view better though I disagree with it being the safest method. The only information a buyer needs to provide is a name and the account number for transfer. I doubt there are very many sellers that would transfer money first. Plus, the recourse for a seller would be much slower and more expensive.
Once points are transferred from Seller to Buyer, the Seller really has no way of getting the points back if payment is not completed. Even if this went to small claims court, I don’t think the court could compel Marriott to return the points to the original owner, though I guess On the other hand, if payment was completed first, the buyer could contact their credit card company for a chargeback.
Either way, it comes down to trust between the buyer and seller.