Next to your marker, IMHO this is the most important piece of equipment you can own for participating in the sport.  If you don't have a marker, there is no need for the mask.  This is the only reason I feel that the mask is 2nd.  The mask not only protects your eyes, it protects your entire face. This is not a piece of equipment that you should get cheap on.  Injuries from a paintball hitting your face unprotected or with substandard protection could be grave, possibly resulting in the loss of your eysight.  Most people will agree that your eyesight isn't worth gambling with.

One of the things I would recommend with buying a used mask is plan on replacing the lens yourself immediately.  You have no way of knowing how the mask was taken care of.  The lens may not have been cared for properly and as a result may have been weakened.  This could make the mask even more dangerous as the flying pieces of polycarobnate from a paintball shattering the lens could be more injurious than the paintball itself.  A paintball is capable of velocities approaching that of a .22 calibre round. Many used masks are sold w/their lenses in place, and they may be ok, however, you need to ask yourself, is it worth the risk if I'm wrong.  Obvious signs to watch out for; micro cracks or "crazing" ,  discoloration of the lens, scratching on the inside or outside, chips, indications of paintball hits on the lens, etc.  Any of these things could be an indication that the lens needs to be replaced.  Aside from the obvious, (like direct paintball hits to the lens), other things that can affect the lens performance and safety: extreme temperatures (like leaving it in the back window of a car), using chemicals to clean the lens (windex, etc), and leaving paintball paint on the lens for extended periods of time.  All of these things may not leave any telltale signs of abuse, but they will have compromised the integrity of the lens nonetheless.  Lenses also should be replaced annually with normal use, due to normal wear and tear.  Due to these factors, again, I feel that the best course of action with any used mask purchase is to presume the worst about the care of the lens and react accordingly.  A replacement lens will generally run from 8-20 bucks depending upon the type of mask you have.  Is it worth spending an extra few dollars a year to protect something as valueable as your eyesight?  That's a decision only you can make for yourself.