Actually....
"WHY NOT PLUG IT IN AND SEE!?"
Actually, that can be dangerous to the person, and harmful to certain appliances. If there is a bad ground in the machine, or bare wires, it's possible for someone to have, shall we say, a shocking experience.
With anything that has electronics in it, powering up something with bad capacitors, for instance, can ruin other parts that are often very hard to find.
We have to remember that not everyone has enough expertise with electrical and electronic stuff to determine if something's safe to power up. However, most people have a desire to live, so if they're erring on the side of caution, I can't say that I blame them.
On the other hand, if something is untested and the seller is not willing to test it, then certainly the price should reflect that.