I'm not sure what you mean by a "module" or where it would reside? It seems to me there are (at least) 3 possibilities:
1. It is a module that would remain on your server, and they would have access to it (rather like my "ew emulator". In which case you can reduce the visibility of your code by implementing as much of it as possible server-side in PHP. No doubt there are people who could circumvent that, but I suspect that you knew that anyway?
2. Is it something like a shopping basket that other developers can plug into their own site? Which you are hoping to sell to many developers, whilst you still own the copyright?
If so, you have a problem. I don't have an answer for that!
3. Or is the person buying the exclusive rights to the code? I.e. You would no longer own it? If so, as others have said, protecting the code is inappropriate, even if you could!
As I understand it, if you compile the script, there is little or no guarantee that it will work on their web host. Also, it's very unusual to sell someone a closed source script, and you run the risk of the buyer being slightly peeved at you. In any case, you might try something like this, but don't say we didn't warn you.
The digital finger prints can help in easy detection of the source of the content. There are some easy to apply tools that can enable this if you wish to apply this for your blogs or site for the Google site ranking.
Use ionCube to encode the PHP code... you can even sell license files for the code and when the license expires the site goes with it until you issue them with another license file! Here's a free (but not as good) compiler instead: http://www.roadsend.com/
You can only really obfuscate the JS code. Don't bother hiding or doing anything to the HTML.