What the heck is this spoiler thing hiding posts, I see it in the market and some threads?

It's a pretty standard forum function. It allows users to collapse long text (so as not to annoy other users), hide plot points when discussing narrative media, or use for the punchline of a joke in reference generally to the other uses of the button.

Huh,, I've never seen the option used before,,, sure looks like where it's popping up it makes no sense,, I doubt in this case when selling an engine the seller used it intentionally