Buying a used suspended leather saddle is always a crap shoot. Buying untouched on-line is not wise, in my mind, unless the seller offers a return policy. Even then, I would pass. This Pro sat in a small wooden shed, in the dark, summer, fall, winter and spring, for many years. The saddle is toast in my mind but it was given to me, along with the bike it came on for five bucks at a yard sale one day...
Look for sag, cracking or tearing leather, pulling away from rivets, impact damage, distortion (I dress left, so tip my saddles a wee bit to the right) and, of course visible impact or scuff damage.
If you do buy a leather saddle, you better just go an buy some leather preserver of some kind. I use Proof Hide on my leather saddles. And, if you do get a Brooks Professional, be prepared for some butt discomfort until the saddle conforms to the shape of your butt. And, with a Brooks Pro, the leather is thick and does take a while to break-in but once it does break-in, the comfort factor goes up. That said...
Not every saddle, be it suspended leather, or fabric or just plain plastic, will be good for every butt. My personal riding favorite saddle does have a leather top but it is not suspended, does not require any form of tensioning, does not need to break-in and does/did not last forever. But it did help me cover thousands of comfortable miles before the leather, saddle frame and appearance took a hike...