Have you taken a look at craigslist and looked at local competition from small builders like you? I see lots of local gaming builds using AMD FX around $500-600. You might have to determine a unique niche.
S&H will eat your profit unless you have a shipping arrangement. I sell books online and I don't sell internationally below a certain price point since it's eats over 25% of my profit.
If you do this, you might want to stock up on parts when they come on sale. Things like optical drives, HDD/SSD, fans, cpu coolers and other items that don't need to match other components. OCZ has been liquidating old stock recently. I think the other SSD makers are also doing that. Just stress test everything you get to save you headaches later on. Be prepared to tie up a bit of your capital in assets.
Be very kind to your customers and provide above-average service. The only way you get business is through word-of-mouth or through reviews. Good Luck.
Just my $0.02.