One of the reasons I love the EVE Market is it means stupid people can be punished.
In most other MMO's I've experience, you can't really do buy-orders - you can however buy stuff at someone elses pre-set price (and in some cases participate in a auction).
Recently the corp made 35mln isk from one persons stupidity, buying an item we had priced at 450k.. by what it seems was him placing a buy-order for a silly price (probably trying to place at 350k and adding two 0's) - or by him trying to buy a more expensive item on Market. I only wish the guy had brought more than one of the item in question - as 35mln isk for an item that costs ~250-260k to build is good business :)
I've mentioned before, but I also will punish anyone pricing an stack of items for below their mineral worth (its gotta be a stack to be worth me flying a hauler over for!), as with my 0% loss on item refines I'll take the cheap minerals thank you very much.
But how do I figure out whether its worthwhile? Simply put I work out the cost to build an item with perfect ML level with mineral prices that are 15% lower than buy-order rates for the region I'm working in... for this I use the brilliant EVE MLcalc tool which is also my main tool for finding out if a blueprint is profitable or not(based on current market prices).. If the price of an item is lower than this, its 15% cheaper than buying the minerals themselves... so I'll buy the stack.
I have 2 copies of the tool installed, one updated 15% below price of mkt, and one 15% above. This allows use in both building (with a safety margin) and for getting cheap minerals with the aforementioned setting. The 15% margin I put in means I rarely need to update MlCalc with daily updates - and also means I know I'll be buying minerals 15% below what they usually cost in the refine example...
Carebears/pirates can also use this tool to find out true costs for items they buy - so when negotiating on items player-player, it can be used for this vital information.