I need a battery for my mobile phone and needless to say my first port of call was eBay. There are many different options, but fundamentally most seem a lot cheaper than they should be. Some are as low as $10, but the real price is closer to $60.

What's the difference?

Well, unless you are very very lucky, the cheaper battery wont be genuine (no matter what the seller says), and probably wont charge or last more than a couple of hours. I know this from bitter experience - twice (once with a battery for the camera, the other for my phone).

I should know better of course, but when you see a good bargain, we are almost conditioned to leap at the opportunity.