Shopping for French Doors
Is this an exterior or interior door?

If it's exterior, I'd recommend going with new so you get a good weather seal.

For an interior door, if you can find a good quality used door in a style you like, I don't see a significant disadvantage to getting a used one other than the trouble of making sure it's mounted correctly on the hinges.  If the door frame is included, you'll want to be sure the depth is the same and that it hasn't been cut too short for your flooring.