My doubts about this kind of "system or "knowlegde" selling are always the same. Why would someone be profiting thousands on a business, wich as stated by OP comes with some workload, and then uses is time, which is not infinite for sure, promoting and selling something for a few dollars...
I already answered on similar question at my website (which currently is not connected with the domain correctly, but I will try to sort this out by Monday). However, I will answer here too.
Few years ago one person asked me to be his mentor for domaining. He was prepared to pay very good but I declined his offer because he wanted me to be available on Skype and webcam. I don't use Skype neither I don't want to install, and I am mostly on my PC which is without integrated camera and I am not a teenager who fancy to show himself to a girls in chat rooms so I don't want and don't need a cam on my PC. More important, I live my life on my own way and I am not interested to be available when someone tells me I should be available.
After I refused his offer he made a new one. He wanted me to write him a detailed guide in which I should explain exactly what I do (how do I trade domains). He offered good amount for that written guide and I accepted his offer and wrote him a detailed guide. That moment my guide was born. At the same time I was developing a website about domaining on my native language. Since I created this guide for mentioned person I get the idea to include it in my website and start charging small fee ($49) for it. Sales are low, but I don't mind as my main intention was to present domaining to my local people who mainly work at some shitty jobs for mainly up to $1,000/month, and often even for less ($700-$800). I also wanted to have my own website where I can write about domaining. I wrote dozens of long articles and published them at my website. Later I implemented a blog section where I from time to time publish some new blog post, mostly related to domaining. It is some kind of my hobby. I like to run this website because domaining is my passion. When I make a sale of my guide through my website I earn additional cash. It is very low amount, but why to refuse it when I already have my guide written and I dont mind answering on question of people who purchased it as I like domaining and often talk about it for free. I have huge knowledge about it so it is not a problem at all for me to answer any question they may have.
OK, now about this thread and sale at BitcoinTalk. I like Bitcoins. I heard of them first time somewhere in 2011 or 2012, but get really interested when 1 BTC was worth about $350-$400, in 2013. Later that year I registered here. I don't do BTC mining, and since BTC is still risky investment I prefer to earn it rather than to buy it. Of course I invested some cash in it, but I treat this investment as a long-time investment and those Bitcoins are on several addresses which I don't touch, and I plan to hold them at least for the next few years. I have many other addresses which I use for different purposes and I like to get new BTC (or Satoshies) from time to time. Selling my guide here for BTC is good solutions for me to get new BTC without buying it.
I had three big business passions in my life. Those are stocks, bitcoins and domains. I don't trade stocks much lately, and I bought BTC in the amount I wanted. However, domains and bitcoin are still my business passion. I trade domains for a living and I like checking the BTC price, just as collecting them here and there. That is why it is not a problem to me to write here explanation why I am selling my guide here. Today I don't work with domains and have time for this post. Actually, I don't work everyday at all and that is why I love domaining so much. Sometimes I work 8-10 hours per day few days in a row, but sometimes I don't work at all for 10 or more days. I am not greedy, and when I make a good sale I often choose hedonism instead of striving for more. As long as I make enough to cover all my monthly expenses and have enough money for fun I am satisfied.
I would not be selling this guide if I would need to write it again every time I sell it, no way. Since I already have it (I only updated it since I created it) and since I have domaining knowledge in my head I don't mind to sell this guide and make some extra BTC.
I hope I managed to answer your question.