I think before you jump on Zigner, who is trying to help you, you might consider that the entirity of this convoluted mess can only be seen from your text and, and as you described them, your "rushed with what to say" words. Advice doesn't always come with a pleasant smell, and no matter how right you may think you are not everyone will always agree.
Now for my "Rough Justice" view of this mess. First off I think you should have never gotten into any messages or PM arguments with this guy AT ALL, period! All you're doing is providing him with the evidence he needs to beat you in court. Once the judge sees your "I don't care" attitude you're gonna be screwed!
It's truly starting to sound like you sold him something different than you advertised it to be, and no matter how minor you may think the difference to be, it was a difference that this buyer didn't want, and when this happens it's considered to be "Misrepresentation." Did you do it on purpose? I don't know, I hope not! Never the less it happened and this is when the 19 year old in you needs to awaken to manhood and stand up and do what's right. Refund the mans money and properly list the item as what it is and resell it again. That's the right thing to do. You don't sound like you will go that route, but you came here asking our opinions and that's mine.
I'll say this much JT_Detailing, from what you've shown us that you've said to this guy, and you outright stated, "I didn't know their was any difference with the bowflex I had posted!", that's just not a viable defense! Misrepresentation is misrepresentation, and if what you sold him wasn't what you advertised then you're the wrong party here.
Secondly, no matter what route you take, if you're not going to refund this guy his money you need to, at the very least, stop all communications with him. Right now if you're arguing with him all you're doing is providing him with ammunition to use against you. I mean why not just give the guy a gun and toss him bullets one at a time and see how many shots it takes to hit you. That's virtually what you're doing every single time you speak with him, be it by PM or emails, or anything. Any future communication with him is only feeding his benefit by helping him prove his case because of your attitude!
I will also tell you something else also, and I'm not trying to be a Smart Alec, and this comes from my nearly 56 years on this earth, the one thing I see from your posts is that you totally need to get a grip on that attitude, you need to calm down and think things out before you react. You came here for advice, but no one ever gets advice 100% in their favor, we all have different opinions and different ideas, but I, for one, am not going to sugarcoat the truth for you, you asked our opinion and that's what you got. There's an old saying I use all the time, if you don't want to know the truth don't ask my opinion, because that's exactly what you're gonna get.
I fully believed you to be totally in the right when I first read your original post, but when I see things like "Take me to civil court, this is a 'as is' transaction. I didn't know their was any difference with the bowflex I had posted" I can only suggest you pay the man his money back or you may end up having to pay him three times what he gave you just out of punitive damages, mainly because of what's cominng out of your mouth and into your messages to him. Your "I could care less" attitude won't please a judge I promise you, it'll only anger the court against you. It's hard for me to believe that you don't have paperwork showing you which model of this Bowflex you purchased, and if you misrepresented it to someone who took your word that you were selling something other than what you actually sold, I think you're going to find yourself up the creek without a paddle.
I just looked on the internet and found a Bowflex Sport and the cheapest one brand new I saw was on eBay for $699.00, and Amazon.com had one for $799.00 brand new! There are two versions of the Bowflex Xtreme, one is a regular one for $1299.00 and the version 2 is $1599.00, so there is no way in heck you're gonna convince anyone that this guy came over and bought your used machine for $49.00 less than a brand new one with a full waranty from Bowflex, or $50.00 more than he could have gotten it off eBay brand new. I think he fully believed he was getting a Bowflex Xtreme and once he got the box home he saw it to be something less. It can be argued that he should have opened the box more than he did and examined what you were selling, but he took your word! Maybe you don't realize it now, but by the time you're my age you will have fully realized it, there's not much more important in this world than your word and your integrity.
Good luck,
SnowCajun