When buying an e-commerce site many times the site's owner will list all of the profit whilst holding back information about the number of refunds that have been issued. This is especially apparent with informational products like eBooks.

Always request full revenue screen shots from the seller. It's fine if they cross out the identifiable customer information, but if you see entire lines missing from their screen shots, more often than not that's a refund entry.

Now, that being said, a refund here and there is normal, in fact it's probably better to see them sporadically than not at all. But before you invest in a site you need to dial in an accurate rate of refund. Without it you're bottom line is up in the air.