Interesting that round numbers mean lower selling prices but that the sales happen quicker.
Steve Tadelis is one of the co-authors and he's done some interesting research from his time at eBay. Last year he published a paper that paid search ads were not really effective and that made the rounds, even getting mentioned in The Economist. Google wrote back to refute the claim, saying eBay is a special website and search ads do work. Here's that study: http://newsroom.haas.berkeley.edu/research-news/study-finds-...
For fun you can try out non-integral factorial values. For example, type in (1/2)! into WolframAlpha or Google, and you'll find the answer is the square root of pi over 2.
Thanks. I can assure you the graph is not fake--I would have make a much closer fit if I had concocted the data. The bell curve fitting was done by hand to be suggestive, sorry I did not do an algorithmic fitting (perhaps I should have done more research on this).
I have written ebooks and was considering courses, so let me offer an opinion too. I opted to develop YouTube videos to promote my brand MindYourDecisions. The reason is YouTube has many more users and you have a chance to establish your authority and market books. If you get over 10,000 subscribers on YouTube, you can also make paid subscriptions which you can use to sell courses.