And this is why I always stick with selling on Amazon. Post a pic, list at $5 under the lowest (inflated) price, and be ready to ship the next day. Never had a single complaint/refund request and I always get paid.
I'm guessing power supply. New or not there are a lot of crap quality PSU's on the market. What brand/model of PSU is it specifically? There are several members on OCN who oughta be able to help get this sorted for you given more specific info.
I believe, in Texas at least, you are only charged sales tax if your order is shipped from an Amazon warehouse in your own state. Some of my recent Amazon purchases have had sales tax and some have not.