Depends entirely on where you're getting parts from. If you're looking at places like EBay or Craigslist (or even Tom's Hardware Classifieds), there is always a risk of a "slightly used" component having actually be run through the ringer. There's just no way to tell who is being honest in the description of the used component.
It's for this reason, that unless I just need a "throw-away" component to tide me over for a day or two I only buy new or refurbished directly from the manufacturer.
The only "used" component I ever purchased was a "Scratch and Dent" laptop from Dell. It lasted three years before one of the RAM slots died. About six months later, the wireless card failed. I installed an USB wireless device and it ran that way...