I'm looking for a second PC, and keep having trouble finding a decent price for the kind of stuff that I'm after. So, out of curiosity more than anything (I'd already checked dozens of sites), I looked at eBay.
The PC's, home-made for the most part, are selling at around half the price of normal PC's. The only difference is that they are personally built (and this can actually be a good thing if they do it well, and it looks like some advertise testing of PC's before shipping). They have technical support and warranties, although it looks as though you have to ship the PC - but I don't have a problem with that as I can get help from my friends to fix anything.
Otherwise, the sellers generally have 99-100% feedback and have been around for a while. The prices are insanely low in comparison to the prices you'd get at your local major electronics or computer store. So, would you buy a PC from eBay?
Note: I'm talking mainly to the UK people here, I haven't checked the US pricings. UK pricings are generally a bit heftier after you convert them over, we have to shell out a bit more on retail items.