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".
Student or Learner