Buying 2nd hand PC components is not a bad idea, it depends upon the quality of the product under consideration. XFX used to be one of the leading Gfx card manufacturer couple of years back but now their product quality has been deteriorated a lot and the after sales service is also bad in most of the areas of India. So avoid XFX products.
I've mentioned in one of my earlier posts that a new XFX HD 6670 is priced Rs 4,700 in Kolkata. So for 1 month old card, 4.5K is a good deal.