I'm all for the enterprising individuals. Too many lazy goats out there. Here's my $0.02, some of which has been mentioned by others.
-Have some capital handy and ready. When you buy the parts upfront, if the client decides later they don't want it, you're left footing the bill (it happens).
-In addition, maybe you have a full set of parts standing by, then you order replacements on each order "in advance" of the next order. Meaning, even before you have any clients have the parts for one computer ready to go. When someone orders it, you order the next set. This will keep wait times down.
-CYA. Make sure everything is spelled out. Put "non-refundable deposit" in nice clear print, maybe have them initial it. If there is any warranty (or not) have them initial that too.
-Spell out what is or isn't included. If it comes with Vista, say so. If it comes with linux, say so. If there is no OS, say so.
-Know your clientele. Are you going to get a call a few days after you deliver asking "Hey, there's no MS Word on this thing" (had it happen) and then you have to explain, "No. That is extra. It isn't included with windows." They get mighty pissy finding out they have to pay extra for something they thought was standard on every machine.
Expectations:
-When is payment due? have a clear schedule... 50% up front, 50% on delivery? 100% on order (that's what dell does when you order online)?
-Upon payment, when is the computer due to the client? Again, have a clear schedule. Within 1 week of payment? 2 weeks? Keep in mind shipping times and building times.
-What can the client expect in term support? Will you go to their house? Is it dropoff/pickup only? Do you warranty parts and/or labor? What if they spill coffee in it?
-What kind of performance can they expect? How do they know they are getting that kind of performance? Benchmarks? [True story - I built a system for a guy, who wanted a "reasonably fast" machine. He was coming from a P4 1.something GHz. All he does is browse the internet and check email. I put together a nice box for him based around an e8400. He was put off because he thought the machine would be "much faster" than his old one. He thought it would "almost read his mind". It was much faster than his old machine. But really... how much faster can you open a firefox tab?]
Advice:
Pick a niche, know that niche, and market to that niche.
If you're going the office route, they're going to want cheap computers, but will be willing to pay for ongoing support (there's lot of money in that, btw). Be prepared to go door to door in an office park to sell yourself.
If you're looking at gamers, they will pay extra up front for a high-performance machine. Then they don't want to see you again until it's time for a new one. If you're near a college, there must be a gaming club or two on campus. Find them.
Treat yourself like a system builder (like Dell, HP, etc). I'd recommend you have some standard warranty, like 30days parts and labor, then 90days labor only. Give folks the option if they want to pay for additional warranty like 1 year parts and labor. They are less likely to consider you a back-alley dealer if they have a sense of security in a warranty, it makes people feel better and more likely to buy from you than if you tell them "if it breaks you're on your own".
On your flyers, don't tie yourself to any particular part unless you need/want to. Definitely be specific about the CPU and maybe the motherboard, but any old 2.1 speaker system will do the job, just find whatever's cheapest. Ditto for the DVD burner. Instead of specifying PowerColor Radeon 4890, just say ATI Radeon 4890 that way you can get another brand if its on sale. Unless that specific model is a selling point, don't be so specific.