eBay for the CPU and RAM, Amazon/NewEgg for the Mobo and PSU. Don't buy from a "best seller" as they're probably a reseller and clueless about the item they're selling. If you buy from eBay make sure you are buying everything at the same time and testing it/retesting it to make sure it's all working. Run it for a bit of time, stress test it for a few days, if all is well and no errors, odds are you will not have any issues for quite a while. I'd say that 99% of the errors I have encountered from computer parts after arrival will take place within the first few days if not the first few hours; this means that you have probably about a 99% chance that your parts are in good condition if you receive them and they're in working order during this period.
Also, for reference, the parts that will encounter the most issues are the Mobo and PSU as they are not only critical components, but also house numerous critical buses, individual traces, etc, and if even just one is messed up from improper care due to previous owner, which can result in an unhappy you; that is why I recommend buying these from trusted sellers. Quck edit here, the reason I didn't say to buy the CPU from NewEgg is this: it is a critical component with critical internal components, yes, but it is pretty much shielded from what your average moron would do; in other words, if it is used, most of its time was probably spend under a heatsink, if it wasn't, then it was sitting on a shelf or in a box, and either way, you can send it back if it doesn't work.
As for the CPU, go with the 4770k. I can tell by what you're buying that you are wanting something overclockable. If you want the 4771, someone in reviews said it was good for virtual machines, but you'd need to ask someone else other than me.