Why does it have to be an advance and not just a regular credit card transaction?
Because unknown people on the internet using CC/Paypal are generally unreliable.
Exchanges that had PayPal transactions for BTC got burned due to high fraud rates and chargebacks.
There is even a warning on the exchange forum not to use credit card/paypal to BTC conversions.
http://forum.bitcoin.org/index.php?topic=14632.0They are too easy to cancel so that you get BTC but the guy you paid to, gets nothing.