So say I had the max amount of money, then I sold a blue flute to the maniac in Riches mansion. Would I just give out the flute for free? Would he refuse to buy it?
Nothing, it will just add up to the 9,999,999 until you waste the money for it to decrease. Note, you will not see the add up as the Max Money is 9,999,999, but it is there.
Source: Tested