What is this a used computer that you are wanting to buy off someone ?
If you were to buy the cpu , motherboard and video card new then you would be paying around $450. Add to that the OS goes for $100 and then your left with the ram , psu and hdd. So now since these parts are used you have to take some depreciation and I would say that for $450 you are getting a pretty good deal but if you want to squeeze some more that's up to you , the deal as is does sound pretty good. I hope it's from a reliable source.