i'd never buy a used clutch. just because it looks okay, you dont know how the previous owner drove with it. most idiots celebrate a new clutch by doing a nice smokey burnout. IDIOTS. you should take the few 1000km or more nice and gentle to let it break in, so it doesn't just glaze over.
It's already out, might as well replace it with something decent. Puting an used clutch in is like puting used brake pads on - Sure they're cheaper, but in the long run you'll more than likely end up spending more than you could have in the first place on new stuff.
A used clutch might be alright if money is tight and you're someone who can install it yourself. Though I wouldn't pay much more then $30 to $40 bucks. Gotta remember, the clutch is a pretty critical part of you're car. Don't skimp and buy junk. Save your pennies and get a new one if you can.