Its up to you... If your building a whole new rig then i would buy brand new because you know its a good part and anything goes wrong your covered. With buying 2nd hand its sketchy because it may have been abused or damaged.
If your just looking for a cheap upgrade from your old parts then id understand. Me personally i wouldn't put used parts into my computers. I would just save up the extra few bucks and buy at a later date so you know what your receiving is first quality.