Well, if there is a period that your transaction might get reversed, then the seller can't send you the stuff. Or, maybe you need a reputation system for buyers and sellers and so on (what eBay does) along with chargeback systems figuring out whether to let the transaction be reversed.
Typically, that period is until the next consensus. In Ripple, the consensus eventually takes place but sometimes may take up to a minute. In Bitcoin it may take 30 minutes. You are never quite sure that everyone "phoned in", but presumably if the network participants announce themselves on the network then you can know when a supermajority has "phoned in". If they haven't - you've got a fork baby.
You can also get a fork if they phone in but start a second consensus somewhere else lol.
