"To look a gift horse in the mouth" is to receive a gift, but then complain about some aspect of it, or maybe even complain about the entire gift. Imagine being given a horse for free and then inspecting its mouth for problems, as if a free horse is worthless to you if it doesn't have a perfect mouth.
A "house of cards" is anything that can fail easily and, when it does, it fails big. For instance, if someone tried to pretend to be a rocket scientist to win over someone, they would probably be found out quickly and then lose any chance of winning this person over because it would also become obvious that they are a liar. The phrase is often "to topple like a house of cards," because it's difficult to build a house of cards without it falling.
To take something "with a grain of salt" means to not let it influence your thoughts very much. It's often said if someone makes a claim or gives you advice on something that they're not an expert on. For instance, they might say, "You are probably going to need surgery, but I'm not a doctor, so take that with a grain of salt."
A "flash in the pan" is something that seems valuable momentarily but turns out to be worthless. For instance, Beanie Babies might have seemed like a good investment at one point, but now they are worthless. It can often be said when talking about fads that involve money. The idea comes from gold miners seeing a literal flash in their pan that they think is gold but then soon finding out that it's something worthless.