since you mentioned craigslist i'm guessing usa? its a dead end.
you might make about $50-100 at most while having to put forth quite a bit of money up front. its a big risk as if it does not sell quick its money lost due to depreciation. in addition you get people trying to chew you down on price, expecting price matching or some form of warranty. not to mention do you really want strangers in your house and to deal with weirdos? plus, given the value of the commodity its also a target for theft.
most people, even high end customers, are completely clueless about parts choices and what is good. a person might google that an i7 and gtx1080 are good but will completely overlook everything else. those who do know parts in and out will...