Supposedly, capacitors can go bad over time. The flash sits there and whistles, but never reaches charge. If you know what to do, the capacitor can be replaced.
Now here's the funny part: the only capacitor I've seen go was one in a flash I owned,
a Vivitar 283, when I didn't use it for a few years. Since then I've inherited an 80s-vintage Vivitar flash, and it was fine, had two 80s-vintage Nikon flashes come with cameras I've bought, and they were fine, and bought one other Nikon one because it was cheap, and it was fine. Another Vivitar 285 I bought 15 years or so ago is fine.
At this point, I'm up to my ears in flashes that I mostly don't use, and I'm inclined to say that the risk of buying a used one isn't too bad.
I'm with charjohncarter on choice: the only one I actually use, I use it because it has a head that tilts not only vertically, but horizontally. Essential for on-camera bouncing for both vertical and horizontal, or off walls in both orientations, which are, IMO, the only ways to use flash if you aren't going to take it off the camera. Also, get one for which there's a Stofen diffuser available, since that's another handy accessory if you use wide, wide, angle lenses and can't bounce.