Most online retailers are keen to keep their customers happy. Unfortunately, postal services are not known for their reliability and sometimes stuff goes missing during transit. When you put these two facts together, it's usually not that difficult to score a refund by claiming a shipped product never arrived on your doorstep. The way you use this knowledge is up to you.
Now, I want to stress that this is just a general observation, and should not be taken as advice. Deliberately lying to online merchants for the purpose of receiving a refund is considered online fraud. It just happens to be extremely difficult to prove and usually isn't worth the merchant's time or effort -- especially if they're based overseas.
For smaller-ticket items like DVDs and books, most online retailers are willing to wear the cost to keep the customer on side. Some will deposit a refund into your bank account while others will offer to resend the same item. Which can then be sold on eBay.
A dishonest person could potentially claim multiple refunds from different retailers (first ensuring they're not affiliated with each other, of course.) Usually, the worst-case scenario is that your claim will be politely rejected.
Again, we're not saying anyone should do this. Online retailers have families to feed, y'know. Don't be a fraudster. No, really.
This post is part of our Evil Week series at Lifehacker, where we look at the dark side of getting things done. Sometimes evil is justified, and other times, knowing evil means knowing how to beat it. Want more? Check out our evil week tag page.