This is possibly the most clever scam out in the universe of eve for the amount of time it requires to set up. I've seen someone pull in 10 billion isk in one weekend minus the cost of the stuff he was using. The scam involves abusing the Margin Trading skill and finding items that are never traded. It can be done with dog tags, overseer effects, etc. You basically want something that literally has no market, and for our examples we will use a imaginary item just called DNA.
This DNA is absolutely worthless, but there is very little of it in the game. The hard part of this scam involves getting between 3-5 of the scam item, but it can be done. The person will then create a buy order asking for 3-5 (depends how many you are using for your scam) for a ridiculously high amount. This makes it look like the item is worth a lot of iskies, when it is worthless. The scammer can get really creative and use multiple buy orders and even put some up for sale at the high price. The goal is to make it look like the item is worth a LOT.
This is where margin trading comes into play. The scammer will intentionally create one or two orders per character and then transfer the rest of the isk off the character. This means that if anyone tries to sell to his buy order it will simply fail, and to prevent someone from messing up his scam he will have min volume set to 3-5. This is to prevent someone with one of the item from canceling all his orders.
The actual scam takes place on the contract market. The scammer will create a contract asking for a lot of isk for there worthless item, and spam the contract in local. People will click on the contract and generally have no idea what the item is or what it is really worth. They will then view market details and see that it is selling for hundreds of isk, and that by accepting the contract they can sell to the buy orders and turn a profit. The problem is the buy orders will disappear as you sell to them and leave you with a worthless item.
I guess the tip of the day, all contracts in local are scams. Stay away.