You'll likely get several opinions about buying used gear, and arguments about life support equipment etc. I'm firmly in the camp of supporting the purchase of used gear, especially gear which can be serviced an returned to great condition. First thing though, age is not mentioned here, but age is definitely a factor with neoprene as rubber can decay over time and use, but generally a good quality dry suit should last 10 years plus, longer depending on how much use it gets and how its cared for.
On the damaged wrist seal, fortunately wrist and neck seals can be replaced, but factor that into your cost. If the one seal is damaged, check out the other wrist seal and neck seal, too. For me personally, I would really prefer to try it on rather than buy it sight unseen, even if the owner is "nearly my exact build." I'm picky that way, but just because someone is my same height and weight may not mean they have the same waist, thigh, or chest size I do. For example, if your thighs happen to be thicker than his, when you're pulling the legs on, could be tight once you add undergarments. If it's sight unseen, maybe you can ask for those measurements too.
To your question, is it worth it for $500? I think age and how much is going to cost to replace the seals give you your answer. We take them at their word that it has 100 dives, which is fine, but if the suit is 30 years old, no, not worth it. If the suit is 5 years old or less, and the zipper and valves are good and clean, totally worth it! I replace my own seals now, but check out how much it costs to have the seals done and see if that's in your budget also.
