Yeah, the nuts and bolts of selling the product is the easy part. There are loads of services out there to sell digital goods including the one that the submitter used and my own service (shameless plug: link in my profile anyone is interested).
PayPal is still the cheapest and easiest way to collect the money, even considering fraud. (PayPal almost always sides with the buyer and with digital goods you have little recourse.)
It's the marketing that makes all the difference. We have customers that have made $50-60k in less than a week due to awesome marketing. And we have customers that pull in well over $100k/yr due to good marketing and good affiliates. We also have many that will never sell anywhere near that much in the product's lifetime, also due to marketing.