I may not know as much as other people here, but I'm assuming that the value of the larger snake is that it's breeding size? Otherwise why are you considering this trade?
On that note, why would the owner be offering a larger snake for a smaller snake of the exact same morph and gender, unless he knew there was something wrong with his?
The nature of this trade just seems fishy to me, but maybe there's something I'm ignorant about *shrug*
My inexperienced $0.02.