i'm having a similar problem, i just made a paypal account, i sent a few payments to this site, and they processed.
then yesterday i tried to purchase another script and i get the error "you must add funds to your account to purchase items"
i then called paypal, they said it's because i have payments being sent out already that are not processed yet.
and paypal pays for the stuff i buy until my bank processes and pays paypal back, so i can only have so many payments out until i can buy stuff again.
plus it's a new account so they are watching it closely for security reasons.
it sucks..