Also, are you going to be selling this out of your house? Because when that power supply blows up they might come knocking. If not, how much of a legitimate enterprise could you run selling PCs out of the trunk of your car in a shady parking lot deal?
If you really think about this your profit per hour isn't that great. You're spending time researching components, assembling the box, installing windows, possibly troubleshooting the system, maybe even doing an RMA, posting in on Craigslist, answering phone calls and emails, arranging a time, going to the sale, etc. By the time this is all said and done how much time went into this? Could you have made the same amount of money by getting a second job delivering pizzas?