With just one slash at end of statement "/s", it means sarcasm.

I got into trouble with this in a comments section, was watching a video summarizing the Great Leap Forward, in comment I stated: "This was much easier to digest than my book on Mao, which is 1000+ pages."
Few weeks later I saw half a dozen replies asking where to buy the book I authored on Mao. I think the repliers were justified in the way they read it, so I added clarification that it was a book I bought. In writing, sometimes you have to be a bit verbose verses speaking because the folks reading can't gain any context from your inflection or expression.