The sites are quite simple. Sign up for a free account, post an advertisement selling an item, and wait for someone to contact you to purchase that item. If you live in the same area as the buyer, you can simply drive over to their house and drop off your item for the price you listed it at. But what happens when you get contacted by a person who lives an hour or more away from you?
Sure, if you want to be hard-nosed, you can charge the buyer an extra price for the gas you'll end up spending driving to their house and back, but I doubt their interest in your item will remain high when they learn they'll have to pay an extra $20 on top of the listing price.
So the next best bet is to ship the item out via the post office. Where the problem lies is in the trust factor. Unlike Ebay, where you can use their online PayPal service to ensure that the buyer gets their item, and the seller gets their money, Craigslist and Kijiji depend on both parties trusting the other to keep their end of the deal. If you're already going through these two classifieds sites, chances are you're doing so because you don't want to pay the many surcharges Ebay and PayPal tack on just before your item sells on their auction site.
Trust. It used to exist in the past, and it still does right now, albeit not as much. If you send out your item prior to receiving the money, how do you know the buyer won't just take your item and not pay you? But on the other hand, if you send out the money prior to receiving the item, how do you know the seller won't just steal your money and not send out what you purchased? Kind of a chicken or the egg problem, isn't it?
Personally, I consider myself lucky. I have yet to encounter a person who has "scammed" me. I work on a "Send me the money, then I send out the item" policy, but I have found quite a few people unwilling to proceed with the transaction, arguing that they've been deceived or scammed in the past by someone else. To be honest, I can't blame them. But these brainless thugs are ruining the reputations of us good guys out there.
Aside from my policy, I also charge about $5 to $15 for shipping the item, depending on the weight and dimensions of the packaging. This includes the standard $100 post office insurance policy, a signature-on-delivery to ensure the person receives it, and delivery confirmation from the post office. Why do I do all of this? Because I still believe in the trust factor - that a simple agreement still has meaning, whether it's written in an email or spoken over the phone. Even in the 21st century, not every person you encounter is a bad person.