jmkidd - That is all you can do. You're approaching this the right way - you had a negative experience, which leads to negative feedback.
Just because you leave negative feedback for a member doesn't mean you can't feel for them or be sympathetic to them going through something. It's not a personal attack on them, but rather a reflection of a certain transaction, and I think that's well understood on this site. I do hope that everything with Shawn is going okay, and I think jmkidd feels the same, but just because he's a good guy dealing with some issues does not change the fact that he did not complete his end of the transaction.
Not holding up your end of the transaction = negative feedback. Nothing wrong with that. It's very possible Shawn will resurface in a few months and be able to hold up his end; that seems to happen with him. And when/if that time comes, even if you cannot amend feedback, I'm sure a message to Admin could allow you that option. But for now, leave detailed negative feedback - as others have said, facts cannot be disputed.
And please, please leave negative feedback for jddavis - the more we can get on there that may deter new traders and save one or two before having a bad experience, the better, because if I remember right, this particular user seems to target first time traders for whatever he can.
I'll never quit collecting entirely, but I am downsizing. Check out my COMC store and help me thin out what I don't want so I can buy cards that I do want. See something you like? Send me a message on here, and we can knock the price down quite a bit. I'll even take a bit of a loss if it means getting you a card you really want.