Excellent condition: basically new and means that the piece was never used, new or near new and may come with an original box that is in unused condition.
Good condition: generally means that the piece is in very presentable state and may have defects other than how it left the factory. It may have some minor discolouration, minor wear and tear to the item, such as small scuff-marks, scratches, and minor dents.
Fair condition: a terminology used to describe a piece that is reminiscent of the wear and tear expected of vintage furniture, meaning it has wear and tear consistent with its age.It could have a patina, or dents, scratches, which would be expected of pre-used items.
Poor condition: scratches, dents not consistent with wear and tear, perhaps need some TLC and restoration. Any piece sold in this state will come with a full description