Honestly building PC's and selling them is going to be pretty hard, you're in competition with companies that have been around for years who offer full time support, warranties and can afford replacements if they accidentally break something. If you bust up a computer that cost you over $1000 then that's coming straight out of your pocket and probably taking the profits of 3 or 4 systems that you've sold. They also get discount on parts which you wont be able to get.
I don't mean to quell your dreams but this is a far fetched idea unless you can get some serious financial backing, if you enjoy this sort of thing maybe look for a job with one of the well known companies or even a local computer shop?
Hell, you could even try building for few of your friends. I've built a few systems for some friends and made a quick buck and there is no worry about any of what I just said above, the only problem is that they will contact you first when they mess something up!