1) the max gold you can have on your char (not counting piggy bank and atm) is 9,999,999
2) if you have 9,999,999 on your char the system won't let you sell anything, even if its for just 1 gold.
3) I belive (from what I have heard and read on forums of wlo) that there is no cap on gold in the atm, but piggy bank can only have 9,999,999.