Here's an article showing why buying knockoff Chinese copies is a really, really bad idea. It centers on iPhone chargers but the same thing applies to any power supply... you don't want to get it wrong when there's mains voltages involved. Ask yourself what corners they're cutting in order to sell you something at 20% of the OEM price. Pay a few bucks and buy a properly engineered device.
http://www.righto.com/2012/10/a-doze...-apple-is.html
And here's an article about a specific charger and why it's so dangerous:
http://www.righto.com/2012/03/inside...d-why-you.html