I think they need to offer a 365 day 100% money back guarantee if not satisfied for any reason.
Then the buyer takes less risk, the seller still knows they are a 'real buyer' not a freebee seeker because pulling out the credit card is still a commitment, even if you decide to get a refund later.
I'd probably just give a 60-day money-back guarantee seeing as 1.0 is set for release around October. By then, if the features you're regularly using aren't ironed out it'd make sense to refund.
Then the buyer takes less risk, the seller still knows they are a 'real buyer' not a freebee seeker because pulling out the credit card is still a commitment, even if you decide to get a refund later.