Conversely, the whole 'buying' metaphor is, of course, related to the notion of someone trying to sell you something. Consider the following conversation: STUDENT: "I was too busy." TEACHER: "What are you trying to sell me? That you couldn't find the time to do your homework!?" STUDENT: "Yeah, pretty much." TEACHER: "Well, I'm not buying it!" 'Pitching' is also popular, like: "What are you trying to pitch here?" (from sales pitch, of course)

I agree with Mark Kramer. 'Not buy something' is an idiom. It means not accept something (to be true). 'You may think so, but I don't buy it.' 'The police wouldn't buy his story.'

"Not buying something" means you're not believing it; you're not accepting it as true. Like: "He said he can speak ten languages, but I'm not buying [it]." You can drop "it" if you want. Or, "He said he can speak ten languages, but I don't buy that." P.S. You can usually also use "buy" along with "into," like: "I'm not buying into the whole the-suspect-just-fell routine."