Depends on who you want to put the burden onto, the buyers or the sellers. If a buyer has to pay up front, especially with a marginal service like this, they're exposed to opportunity cost (maybe they were going to do something else with that money, although interest rates don't really exist for Bitcoin yet) and counterparty risk (why do you trust this website to hold onto your bitcoins and not be hacked?). If this market never takes off - as is one's default expectation - it'd not be pleasant for buyers to have paid up front.