You should shop for a better quality motherboard and get a better power supply too. Buying computers on eBay generally doesn't save you any money. Usually, you're much better off watching the discount sites for a special on the machine you want. For $900, you can get the 17.5" laptops with quad core I7 and all the goodies.
You should shop for a better quality motherboard and get a better power supply too.
Actually, the listed motherboard seems pretty good, it's one of the latest generation Gigabyte motherboards, although a low-end version. What makes it low- end is that is doesn't have all of the features of the higher-end models, such as SLI (running multiple Nvidea graphics cards), although it does appear to support Crossfire (running multiple ATI graphics cards).
Gigabyte is a well established company, and their motherboards are highly overclockable, there's people who've overclocked their I5 processors to 4Ghz with that particular motherboard on air cooling alone.
As far as the power supply, that would be my one main concern, where you have no idea what type they've used in there. For the money their asking for, it most probably isn't a high quality supply. You COULD ask them to put a particular brand in if they customize their builds. Let me know if you need advice on that.
One other thing, the standard Intel cooling fan sucks. You can ask them to put in a better fan, or put one on yourself. It's not that hard to do, but it requires a bit of skill as well as a knowledge of how it's done. Let me know if you need info there as well.