Gah, I don't check the forum for 2 days and this happens!
This is locked now and moved to Open Topic (it's not help/tech support).
To clear up some of the discussion points:
1) AFAIK Prof-UIS does not factor in to this at all, it's just a UI library which can be used in commercial apps too (don't know if Construct being GPL changes this, but I'd guess not)
2) The GPL specifically states it is allowed, but pointless, to sell GPL software. Sure you can sell it, but you'll never get a high volume of sales, because you rely on the fact the buyers don't know they can in fact get it for free. Anyways, no issue with selling it here.
3) The only grey-area issue is the bundling of tutorials, games and artwork from the forum. These are NOT automatically GPL covered as spelt out here, so there is no automatic permission to sell them. In many jurisdictions such as the UK, copyright is also automatic, so you are not required to explicitly state that the work belongs to you or is copyrighted to you for the work to be protected by copyright (eg. artist's paintings don't usually have 'Copyright (C) 2010 ...' in the corner ). However, since it could be argued only Construct itself (which can be sold) is being paid for, and the rest of the content is free on the internet anyway and simply supplied as a free add-on bundle, it becomes a bit of a grey area and I don't really know what the rules are. To be honest, the odd stray ebay auction is not going to get much attention if you complain.
The issue of collecting up free games on the internet and selling them has been around for years, and it's pretty much a right of passage for an indie gamer to find some of their work suddenly for sale somewhere at some point. It's a well-known fact (or should be if people here didn't know) that this will happen. To protect yourself you essentially should just put your full credits (name, website, email etc) in your games/tutorials, a message saying it's freely available at such-and-such a URL, and maybe that if you paid money for it you got made. That should be enough to give the right idea to someone who paid money for it, and maybe clue them in enough to start looking for a refund. Leaving work empty of credits leaves it open to exploitation, and removing credits from a work in order to sell it is a much more serious issue.
Hopefully that clears it up. To be honest, if you see anything else like this, just leave it. You probably can't stop it, and even if you do, someone else will. Start protecting your work.
As for the response, I'd have hoped for more maturity. Vigilante justice is never a good idea, even less so vigilante justice before proof of guilt. Next time try to keep cool. I've specifically edited out a post inciting that the buyer's reputation be deliberately reduced; that's very poor conduct indeed. Please, never respond with dirty tactics or name-calling - it's important our community appears respectful and mature, and we're not doing a great job of that right now.
Edit: In retrospect, I can't see if the product sold actually does bundle anything from the forums, other than comments from people here. Even if it does not, the point still stands, you should take preventative action in case.