Actualy, I would have to side with Jpreston on this...
You sold an intake manifold, it takes BOTH halves to make the intake manifold. If you buy a 2 piece manifold from a manufacturer it is labeled as a manifold (singular) and comes as a set, because one part is utterly useless without the other. Should he have made sure, yes, but as a seller, you also should have clarified since you DID know it was 2 parts and even then didn't specify. You can say Jpreston should have known better, but it can also just as easily be said that you made a questionable deal. Pot meet kettle.
Jpreston said he spoke with a lawyer friend asking for clarification, he did NOT say he was looking to sue or anything else. Just saying "Hi" to a lawyer will cost you more than this transaction, and you do not need a lawyer for small claims court. Which also costs more than this transaction. Nothing wrong with asking, as you have stated yourself.
To me you did tarnish your reputation, but not because of the transaction, but how you handled the problem.
You created a stir over $5? Seriously? You could have bitten the bullet and showed how good you were and came out smelling like a rose. Had you stood up and said "You know what, we both made mistakes but I should have specified. Here, I'll send it out free..." You would have come out looking like a hero. Instead you wait to get your $5 extra bucks from the deal. Is it fair expecting the shipping? Yes, but is it smart from a business standpoint? No.
I do not expect to see this discussion disappear. You had a dispute it was resolved. Others need to know HOW it was resolved. The Sell/Swap is a contract between two buyers, the site only facilitates the communication. We do not moderate it or anything else except facilitating communications, same as if you called on the phone to each other. Would you expect the phone company to be able to help? Nope. If you delete these posts you are moderating and controlling the transactions and therefore putting this site and me at risk.