Not really shady as much as certain innovations mean that some CPUs won't work on some motherboards, to make it easier they just change the socket. There's a multitude of reasons, that's just one of the more frequent ones.
If you brush up on your knowledge a bit you should be able to make simple upgrades yourself. When your friend builds you the computer ask if you can watch, don't pester too much though, just watch and you'll learn a fair bit
Even if it comes to it, I wouldn't recommend buying from Ebay. Buy from somewhere like Aria/Scan/Overclockers, at least then you'll get a warranty as well.
I do seem to remember ATI having issues with WoW, but I thought that was sorted. It was mainly a driver issue, and while nVidia are much better in this respect I wouldn't rule out ATI completely.
Personally though I'm an nVidia fan myself, but sometimes the price to performance ratio leans to the ATI side.
Now then, remind me what your uses are? What games would you like to play? Also what do you need, like monitor/mouse/keyboard/OS etc?
I could throw up some specs for you to show your friend and ask if he'll put it together for you. Or if you feel comfortable look around on google for pc building guides, it's fairly simple, kinda like lego for adults. zxh800 built his first one not long a go, I'm sure he'll be willing to answer any worries you have.
Whatever you decide, I'm happy to answer any questions or if you run in to any troubles