So, if a strategy is being sold for games like poker and blackjack, then it's still a bit good, but on the other hand, I guess another question would be, what is the seller still goes ahead to sell same strategy to other gamblers as well, which i sure trust he or she will as long as it's money we are talking about, this also will completely destroy the whole strategy, so, in the end, we still discover that its a very foolish decision to pay for strategy to win games, be it luck dependent games or skill or knowledge dependent games, for luck dependent game has no other strategy of winning aside luck, and this i believe is also same for knowledge based games as well.
People who sell books that teach plebs about "winning" in gambling and/or sell videos and other forms of learning materials are probably not winners in the thing they teach as well. Or the thing they teach probably was profitable before, but stopped being that and the only way to make profit from it is to sell it.
The actual winners will never share their secrets.