Well that 7850 is a little more powerful than a 6950, which is a little more powerful than a 560ti. The 560ti is probably a better deal. But, it also is used which needs to be taken into account.
My buddy only had the card for about 3 months and cleans his components every month, i think he has done a great job taking care of it and it looks like new. Will the 560 ti be able to max out WoW though?