auction house rng

It's based on event command $B6 that does a 50%-50% branch. The whole Auction House system is in the event dump, as an example this code bit will have 50% of chances to branch to $CB4F33 or $CB4F3F:

Code:

CB/4F29: 4B    Display dialogue message $0A51, wait for button press
               ^ Buy it for 5000 GP.
               ^ I'll pass…
CB/4F2C: B6    Indexed branch based on prior dialogue selection [$CB4F33, $CB4F3F]