I've bought thousands of stacks from a farmer for 5g-6g each. Where server price can rise up to 13g a stack.
This is simply a matter of time (X luck) = profit if you ask me. Knowledge helps ofcourse.

In the past while scanning AH a couple of times a day i used to find stuff for a couple of silver a stack where market price would be atleast several gold, or the good old Frozen orbs for 3g, while vendor price is 5g. Easiest money made.