Bottom line is this. You purchased a used sub. You did the right thing by contacting him when you were not "ok" with the product. You lost all credibility after you put it in your car and used the sub(with your lack of knowledge on proper amplifier settings). The sub more then likely was just fine when it arrived(regardless of coil color), however, you now say it got "stinky" with your "500" watts(which is wrong).
There are no warranties applied when buying used products. It is at the discretion of the seller. I personally would not work with you at this point. You purchased the sub and used it. Sorry, but you are at a loss. Now maybe, just maybe, if you ask nicely he will assist you in getting it reconed(all though I would not).
You are(or at least you claim to be) a college student. Being such, you should understand the working of online transactions and how you are more then just wrong in your claim. Take this as lesson learned and buy from a shop or take the PROPER actions next time.