Ok. I'm familiar with graphics cards for desktops, just had never heard them referred to as "discrete" before. Aren't refurbished Macs supposed to be returned to factory spec?
As far as factory spec goes, that can vary.. They return the computer to factory default settings, but minor things could be wrong that they missed, that could shorten the laptops lifespan, or make it fail. Such as, a slight drop/fall, a spill, a short circuit, etc. Things that the previous owner didn't mention so they could get their money back.
Any sort of electronics aren't really something you want to buy used, but epically computers.