In order to maximize your chances of selling a camera lens, you must list it in the correct categories.

First, who made the lens? 
This is usually easy to see, as it is marked on the front end of the lens body.  Some common brands are Minolta, Canon, Vivitar, Sigma . . . the possibilities are immense.  And to complicate matters, many of the major makers used to use a different brand name for their lenses, such as Minolta selling their lenses under the Rokkor or Rokkor-X brand names.

Second, what camera is it for? 
Generally this will either be the brand of the lens, such as a Canon, Minolta, Pentax, Konica, etc, or it will be marked near the lens mount (camera end of the lens).  Further, in many brands, you must know what the individual mount is, such as a Canon FD or a Minolta MC.  (If you don’t know, there are several other guides that will help you.)

Is it a manual-focus or an autofocus lens? 
As a rule, autofocus lenses have electrical contacts in or near the mount; also, in most cases, manual focus and autofocus lenses cannot both be used on the same camera.

Finally, what is the focal length and the maximum aperture? 
This information can be found on the front of the lens, which will read something like this:

MD Rokkor-X 1:1.4  f=50mm

In this case, MD is the mount; this lens will fit any of the manual-focus Minolta 35mm SLRs.  (This may not be present on any given brand, and probably will not be present on a third-party lens such as a Sigma or Vivitar.)

Rokkor is the brand name; this indicates a Minolta lens.

1:1.4 indicates that the maximum aperture of this lens is f1.4.  This information is present on every lens I’ve ever examined.

F=50mm indicates that the focal length of this lens is fifty millimeters.  Again, this information is present on every lens I’ve ever examined.

If the lens shows f=70-210mm, this indicates that the lens is a zoom lens having a focal length which can be varied between 70mm and 210mm; this lens may also have the aperture given as 1:3.5-4.5, indicating that the maximum aperture varies between f3.5 when at 70mm and f4.5 when at 210mm.

In cameras using 35mm film, the following list should give an idea of the class of lens associated with the various ranges of focal length:

5mm-18mm: extreme wide-angle lenses, sometimes called fisheye lenses, though strictly speaking the fisheye lens must produce a severe “barrel” distortion; that is, it will make square objects round.

16mm-24mm: very wide-angle lenses.  These usually do not have the severe distortion; meaning that things viewed through them look relatively normal, though distant.

20mm-40mm: Wide-angle lenses.  Things viewed through these lenses should look relatively normal.

45mm-60mm: normal lenses.  These record pretty much what the eye sees.

60mm-100mm:  portrait lenses, aka mild telephoto lenses.  These produce a mildly magnified image.

100mm-200mm: telephoto lenses.  These produce a moderate magnification.

180mm or more: long telephoto lenses.  These produce a significantly magnified image; the longer specimens are telescopes.

Understand that these categories are approximate and for cameras using 35mm film only; a medium format camera such as a 120-film Hasselblad will use a lens of around 110mm as normal!

Defects:

Selling a defective lens is bad for your rating unless you have fully disclosed all defects.  Some common defects are:

Scratches: pits or scratches on the glass will degrade the image.  You can see these by examining the surfaces in good light.

Oil: lenses should not be oiled under any conditions!  Oil on the glass degrades the image, and oil on the aperture blades essentially renders the lens useless until it is removed.  Close the lens down to its minimum aperture (largest number, typically f16 or f22) and look at the aperture blades from the front and rear.  If they have a wet or shiny look to them, they’re probably oiled and an expen$ive cleaning is in order.  Oil on the glass generally appears foggy.

Fungus:  if there is stringy, branching or fuzzy white or gray stuff visible inside the lens, it has fungus and is damaged, quite possibly beyond repair. (Thankfully, this is more rare than it used to be.)

Dented filter ring: this one isn’t too serious, except that it usually indicates that the lens has been dropped and may have suffered other, hidden, damage.

Loose mount: this usually means that the lens was dropped with a camera attached.  This is a killer: unless the lens is exceptionally rare and valuable, it is best sold as-is for parts/repair only.  Very few lenses are worth having repaired.

General: The focussing, aperture and zoom rings should all work smoothly without gritting or dragging; you should be able to turn all adjustments by holding the ring between the thumb and forefinger and turning.  (If it won’t turn at all, you may be at the limit of travel, or there may be a lock holding the ring.)

I hope this helps you sell your lens!  --MonteCristoRick