Personally, I don't think buying a used sub is such a big deal.

Sure they take abuse, but that's what they're built for. Things I consider when buying a sub:

* if it's a decent brand, and a decent model
* how old it is
* what it was powered with, what does the rest of the system consist of
* how does it look/sound!
* what sort of deal am I getting!

If it's a sub with a good reputation, and not too old, I'll consider it. If the rest of the guys system is decent, install isn't crap, its a good indicater to me that he's a little more knowledeable about car audio, probably treats his gear a with more respect than the average user.

If the sub looks decent (cone isn't trashed, surround isn't worn/torn, magnet isn't scratched to sh*t), let's hook is up and listen. I've got a few choice bass tracks that I've listened to on every set of subs I've owned - I know when something doesn't sound right. At that point, if the price is right, go for it!

I'm not sure about the rest of you, but I'm usually switching up my bass at least every year. If I can pick up a sub for half of what I'd be paying otherwise... why not? Just means I'll have more money for the next setup, and will be able to recover a much great portion of the purchase price. I've even made money on a few deals!

If the subs you're looking at are just $150 new... well I'd spend the extra $50 rather than buy used. If I can get a $800 sub for $300-400, that's worth it to me. I've bought over a dozen speakers used, and haven't been burned once.

My 2cents!