This is a pretty hard topic to address. I personally never had any luck scamming, but it was never my cup of tea. This makes it a bit hard to write about. I am note sure if this should be a warning or how to guide, but knowing eve players... This will be a how to guide.
Courier ScamsThe point of these is to make it impossible, or highly unlikely that someone will be able to finish a courier contract, and thus resulting in the scammer getting the collateral for the cheap items they want shipped. They accomplish this in three common ways:
Null Sec Courier Contract - The destination is one that you might not even be able to dock at. These stations are owned by alliances and generally they will only let other alliance members and their allies dock. CCP changed the contract system so it WARNS you that you might not be able to dock there, but a sucker is born every day.
Massive Cargo Scam - This scam is pretty clever. The target is newer freight pilots, specifically those who do not have freighter 5 trained (since it is a long skill a lot of people don't bother). Level 4 freighters get 940,000 m3, so the goal it to go over that. The common way is to make a courier contract to haul 8xGeneral Freight Container which comes out to about 960,000m3. Level 5 Freighter pilots can still haul this, but its all about balance. If the collateral is 150m and the reward is 2 mil, then you just need to find one sucker for every 75.
Suicide Gank - Simple Enough. Set a really high collateral, a destination that freighter pilots normally don't go to, pray they use a industrial, and suicide gank them at all costs.