I see a lot of stuff being sold used on forums and ebay. Is there any general rule to follow on how much they should be worth compared to new? Anything to watch out for as far as life span?( Would you buy one 10 years old) Which go the distance?

Well....to the first question, only pay what you think it's worth. As far as life span, the only think I would be afraid of are old subs with foam surrounds. Other than that go out and look and catch you a deal. The biggest thing about it is to KEEP LOOKING, day after day. When you're looking for used equipment you don't wanna miss out on something awesome.

Oh and make sure you protect yourself during the transaction. Ask for more pictures, ask what they've run in the past (do you really want to buy a sub from someone who barely understands car audio...I don't) ask questions about the product. What you're looking for is knowledge. Unless it's a killer deal, you always want to buy from someone who knows what they have and how to use it.