The truth
I don't always sell things on eBay. The fees you have to pay eBay and Paypal can be outrageous. I do have a good amount of luck selling my items on Craigslist and the like. But for those that do sell on eBay, here's some advice.
Keep track of any serial numbers. Take several photos of the item before shipping, focusing on anything that makes the item unique. Even have someone take photos of you packaging the item. It's all evidence should there be a dispute.
Buyers scam sellers on eBay at will, and most of the time, eBay/Paypal rules in favor of the buyer. You need all the evidence you can get when it comes to these disputes. I've won three of these disputes and had the buyer's accounts banned. All three of them were with Playstation 3 consoles as I mentioned above. What the buyer's did was buy my functioning Playstation 3 consoles. After receiving them, they disputed the purchase, claiming the consoles were faulty, which I don't have a problem with. What I did have a problem with was receiving broken Playstation 3 consoles that I didn't send. The serial numbers didn't match up.
This is common on eBay, and it's a good practice to notify potential buyers within the listing that you do keep track of the serial numbers and will only accept returns of items that you originally sent that haven't been tampered with. This generally deters scammers from bidding on your auction.