you buy that? = do you really believe that sort of thing? When you believe someone's story, that someone is actually trying to "sell" you his story, and if you are dumb enough you will "buy" it.

It's an expression that means agreeing to or believing what other are saying.

@Richard: Great explanation and example, thanks, Richard, I'm totally clear! ^^

I'm sorry, it should be "He said this kind of magic potion can improve your ability, you buy that?"

"you buy that?" ia an abbreviated form of the question "Do you buy that?" It means - do you believe what someone has just told, or something that you have read, or an explanation that may not be believable. I'm much shortef than Yao Ming, but I'm a better basketball player than him. Do you buy that?