"I am not going to buy your threats" and "who's going to buy your lies" can someone explain these two sentences to me
This sounds like an abbreviated form of "buy into" something. According to this source (https://writingexplained.org/idiom-dictionary/buy-into-something) it's genesis was the stock market, and it is still in use in that regard. See https://www.investopedia.com/terms/b/buyin.asp. In American vernacular, it's used to indicate one is not inclined to believe the subject referred to has value, so they're not going to subscribe (Merrium-Webster: feel favorably disposed) to it. Simplified, if you saw it in the store, you wouldn't buy it. If you don't buy into something, you don't believe it. A similar idiom is "That doesn't hold water." It's like a pail with holes - worthless.