You cannot extrapolate from the Amazon bestseller list to the public at large. Biasing your sample to only people who purchase (and, ostensibly, read) books is even worse than polls that only include people who respond to telephone surveys.
Unless you can otherwise justify your "majority" argument, this renders the rest of your points significantly more less than universal than you put them forward.

That said, your example includes you performing a greater service for your customers, which falls into my suggested tipping scheme. It lends no support to your position that unused skills are worth tipping for.