|"said it was either the motherboard or the hard drive"|
You need to find a new computer store to deal with. All you needed to do was disconnect the hard drive to determine if that was in fact the problem. You don't need a hard drive connected in order to POST.
What exactly did happen when attempting to start?
I agree with SkipCox about putting on the brakes. If they had to guess and had any experience with eMachines they would have sold you a power supply.