I wouldn't say "I'll buy that" is a slang expression, since it has been around long enough to be considered a colloquialism. It is a way of expressing that a person believes, or accepts, something that has been said. Similarly, "I don't buy that" expresses disbelief, or incredulity.
Like you, I have never heard it used that way, and I am quite skeptical. Perhaps I will see if I can get some more opinions for you.My fiancee was in her 'translation class' at university today, and her teacher insisted that the phrase "I'll buy it" means "I don't know".