I actually do think that the worst mistake anyone can make in gambling is paying for a strategy, when winning in gambling is absolutely luck dependent, although it depends on the type of game being played though, like for example, in card games where skill is required to win the game, then strategy must be present too, and the one with the best strategy will always win, like when it comes to games like poker and blackjack, I believe this games are not totally luck dependent, but winning also depends on one's skills, and skills often go hand in hand with strategy.
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.
Strategies that work for professionals in poker, are combination of skills that those players have, and because of experience that they have acquired.
So when i implied that there might be working strategies for poker, i meant that some people have skills they can apply to poker. And knowledge itself isn't going to be enough for working strategy. You have to be able to read the room to apply the knowledge, and even then anything isn't guaranteed to work. It's just too nuanced to sell in a package.
In short: most of these skills that benefit people aren't transfereable, and that knowledge people would pay for, probably already exists in hundreds of free youtube videos.
I totally understand if someone wants to benefit and make money from talking about their experience and tactics in poker, and while it might be interesting to hear, and basics of them would probably benefit amateur players, but paying for that info isn't only way to get it, and depending who is talking, you might not get anything worth your money.