Ya it's tricky. It sounds weird but you didn't include any legal crap saying they are not allowed resell (you have now added it to your topic) so technically the guy is well within his right to resell it. Weird but true.

I'd create a fuss and make the point in his topic. See most people will buy from the original coder thinking they'll get better support. That's about as much as you can do. Their are lots of creative common licences for free. Just add them to the zip file you send anyone next time which will give you more rights and allow you to get the person banned or topic removed which ever forum it's on. You can even threaten or scare them with legal pressure if they don't remove it or take action.