Go here and get their broker bot:
http://www.extremecrafting.com I have tried it, very simple program that uses pixel recog. to do the buying but very acurate.
Problem with buying in stacks is if someone is trying to buy the same stack or partial stack your going to run into issues. It might throw up some flags also if server side it is showing you buying in chunck when your not suposed to be able to.
Loraik