Hi,
I have been told many times that the trade in selling PCs, is pretty poor and you are unlikely to make a good profit off sales.
Things like console, tablets, smart phones are taking away sales from this area and when people do look to buy they will go to the larger stores or online.
Look on eBay at the kind of prices you can sell PCs of a certain spec for then check how much that will cost you to make, there is very little profit in it for the builder.
Anyway if you still decide you want to give it a go here are some places to start looking.
I like in the UK so i buy parts from dabs.com, ebuyer.com,aria.co.uk, scan.co.uk
If you live in the US use newegg.com
Ok so you want to know how to build a PC:
.go to youtube
.go to newegg's youtube channel and search 'how to build a PC' i have fund this to be a good place to start. here is part one: http://www.youtube.com/watch?v=lPIXAtNGGCw
Now you understand a bit about how to build a PC, you need to keep up to date with the latest tech being released, is it good/bad?
check these youtube channel subscribe and you should be covered these guys are good and letting everyone know when stuff is released:
LinusTechTips
OC3D TV
Tek Syndicate
Austin Evans
Techquickie is a good channel for quick guides to do with PC's and building them, just general useful info.
I hope this helps.