Take a look at this image of a public contract on the left and the market info for one of the tags on the right.
At first glance it looks like deal, buy the tags at 540 mil and sell one of them for 871 mil in the same station, a 331 million ISK profit for no work! Right?
Wrong. Notice the rarely used but very evil Min Volume column of the buy order? Set to 6 for the first one and 5 for the second one? What this means is that the first buy order will only accept at least 6 tags at once in order to pay out (and since the max quantity is 6 it means exactly 6). Same deal for the second buy order at 5 tags. The only buy order on market that accepts a single tag is the one for only 7 million ISK. Most likely the contract and the buy orders were created by the same guy who found a rare item with very few or no buy orders and sell orders and set this up, advertising the scan in local to get people hooked.
Even worse, due to a horrible feature (bug IMHO) the buy order could be setup using
So be smart and if a deal looks too good to be true, it probably is.
EDIT: Fixed Day Trading to Margin Trading. Always get those two confused. Day Trading allows for remote modification of buy and sell orders. Thanks, commenters.