This is an interesting question. The word "ploy" is a coined (invented) word. It was popularized by Stephen Potter, in a 1947 book, "The Theory and Practice of Gamesmanship (or the Art of Winning Games without Actually Cheating)." A "play" can mean a brilliant move or maneuver. A "ploy" usually means a sneaky psychological trick that has nothing to do with the actual game play. A marketing ploy means a dishonest marketing trick. For example, a marketing ploy might ask you to fill out a survey explaining what you like and dislike about a product. The company doesn't really care what you think. They just to make you spend time thinking about their product.

You use the word "ploy" when you want to describe another person's cunning plan. It has a slightly negative feeling to it, but it is not as negative as "plot". "He pretended he didn't know how to use a vacuum cleaner. It's his ploy for avoiding housework." "Young children can get into this museum for free. It's their clever marketing ploy to get adults interested in coming to see the exhibitions."

A “ploy” refers to a clever or tricky way of getting somebody to do something. Sometimes it implies dishonest or misleading behavior, but not always. For example: “Buy one, get a second one free” is an old marketing ploy designed to make people think they are getting a really good deal (maybe it is a good deal, or maybe the seller just charges double for the first one).