I sell a few apps on Google's Android Market

On average, around 2% of free apps are ugraded to pro.
Over a third of "sold" applications are refunded.

I have had 1 chargeback which I discussed with checkout support.
The chargeback fee is 5 euro while the app is costs only 1.
After the credit card association investigated, both were undone.

So far I have only used other sites for distribution of free applications.