The first year, I sold for $0 of librairies. In 2014, I've made ~$45,000 and it's 100% passive income. I'm not proud to say that I've worked a total of 30 hours on the site last year (it sounds as if I'm lazy, and I'm not). Moreover, I've never spent a single dollar on marketing, no matter its form.
It works so well that I've taken the decision to leave my daily job to work on the site full time. I (perhaps naively) think that if I make that much money while doing practically nothing, I can surely make a ton more by actually working on it every day for a year. On Feb 1st, I'm making the jump.
It has been tough to get there though. The first year has been a disaster. I nearly abandoned the site. Then, one day, I started to gain traction. To this day, I have no idea why. Then, months after months, the sales went up. It took me weeks and weeks of work to create the libraries I'm selling today. I also did a lot of variations, based on the feedback I received from my customers. My customers are the best, I think. They like what I do, they give me a lot of feedback. In the course of my business, I also did stupid things I regret immensely, like copy a competitor (but honestly it was not intentional), and I'm really, really not proud of this.
Sales have reached a peak of $7500 for the month of May 2014.
The site is based on http://jekyllrb.com/ and is hosted on https://www.webfaction.com/, on a 9$ per month plan. As the site is static, I just need Nginx. That's it. GetDPD allows me to collect payments with both Paypal and Stripe.
To let people pay and downlaod, I use http://getdpd.com/. They are fantastic. I've tried a lot of other options and even though GetDPD looks terrible, it's a great product, well worth the tiny monthly cost.
I hope my story will let people know that it's totally feasible to do a great business as a side project. I honestly wonder EVERY.SINGLE.DAY how come it worked for me, but well,... it worked :-)