Well, how long is a piece of string, and how long have you got.
Basically, if its got a moulded on plug and the correct fuse is in it and there is no sign of damage, then you can sell stuff, and I've done so myself, showing it working. However it gets more complicated for charities and shops. I think shops like cash converters and real second hand shops do test for insulation and any problems or if the device has been recalled and the item they have was not modified.
Personally, if you are giving to charity shops, and the device is older than about 10 years or was not bought in this country, best not to do much with it. Things like lights can and do lead a rough life and the wiring has to be double insulated. I mean I have a table lamp. its made of granite and plastic, yet its cable is only single insulated two core. Nobody wants it.
I guess its insurance for small companies and charity shops. they do not have it.
It has been my experience though that the most dangerous things in the electrical goods line is the wall wart. I've had them come apart, go literally up in smoke and flames and the wires to the appliance actually catch fire. In one case a psu managed to weld itself to a coffee table. Another dell laptop supply set fire to the carpet. Brian