I personally know slapinthetrunk and have been talking to him about this situation.
He is a great guy and I can vouch for everything he has said so far. I have seen the exchange of messages and the seller never mentioned the reversed polarity. To slapinthetrunk- I recommend you post a screenshot of all pm's to confirm this to all the members here.
Personally, I feel you should get a full refund and the seller should pay return shipping. That's definetely what I would want had it happened to me. Some people are saying it's partially your fault for not checking polarity first- but honestly I dont' think that's YOUR responsibility when you are buying what you think is a 100% working sub. The huge majority of people buying used subs here i'm sure don't check this before trying out their subs, and they shouldn't have to.
On the side of the seller: I know who the seller is, and have looked at his profile and he hasn't logged in since last month, hence he clearly hasn't had a chance to see/respond to this thread. He does seem to be fairly reputable with a large post/ref count, so hopefully he will get back to us on this.