My friend and I thought about this too and many others have too. The biggest problem is what yomama9388 stated "they will expect you to diagnose hardware issues/perform all repairs". I think the hardest part would be software issues. Someone people will get viruses because they are ignorant about computers, they will blame the quality of the computers.
This is taken from an thread from tom's hardware asking the same question. Link
I looked into this myself a while back. I did alot of research. One thing that kept coming up was that you won't make boatloads of cash on the sale....but you can make money on service calls. As you currently have no brand-power or market visibility, the only thing you can compete on is price. I wouldn't expect to make more than $50 profit for a budget computer, $100 for a higher end build. Considering the time you spend ordering parts, building it, and shipping it, that's not alot, especially if something goes wrong and you get into RMA's.
When somebody buys a computer from you, if they need help/training/upgrades/repairs, you are their point of contact. They will ask you for help, and this is something you can charge for (Don't be a nice guy and do it for free, however tempting it is). You can easily ask for $50 minimum for a visit even if it only takes a couple minutes to fix the problem, more depending on how long the fix takes. If you get the picture....that's as much profit as the actual sale, and takes way less time.
If you just want to do this in your spare time, use word of mouth advertising to start. If you want more business, then don't shy away from paying for advertising. Get creative....advertising in a hard-copy paper newspaper for repair service will give you exposure to people who are less computer savy and need someone else to fix things for them (otherwise they'd be reading the newspaper online right). Facebook advertising gives a huge amount of control on your target audience.