It's a very simple analogy.
If the price is too good to be true, then it's mostly fake items.
Things like pirated DVDs, unless you are looking for a collectors edition, i suggest you buy it from a local eBay seller in Singapore or get it online from Amazon.com
So far the worst kind of seller is those who want to sell brand new laptops/camera for USD200-300. Just ask them a very simple question on where is the actual product. From there itself you will know that it's a scam.