Most likely since people already do this, but there are some issues. Mostly related to support. If people buy a PC from you, they expect you to help them out when they have issues with the PC, and depending on the law where you live you might be forced to replace defective components unless you can "make it plausible" that it is defective due to wrong use/abuse.
One way to go about this would be to make proper custom builds, with cases modded to look great or to costumer specification. This guy used to do that, now does reviews: http://www.youtube.com/user/TimeToLiveCustoms
Some of his videos might be great inspiration.
Lastly, you made a horrible title for the topic, "overclocking pc's for ebay?" or something like that would have been better.