Aside from the fact that there exist many other canvas blend mode plugins, I'm just going to say that it wouldn't hurt to add to your fancy demo page:

    a) browser support? (this still matters to lots of people)
    b) _some_ notion of its exposed api. (what am i getting myself into?)
    c) what happens when i buy this (can i get support, what's the licensing?)

I could go to caniuse.com but because the fancy demo page doesn't explain that this uses canvas to blend an image on itself, i'm sort of at a loss.

isotope.js makes a pretty good case for charging for a jquery plugin, but it not only has a good demo, it also has docs and makes it abundantly clear that you're paying to use this on a client's website (go nuts on your own stuff). I don't know how successfully it works, but it makes as much sense as charging for fonts, so I suspect a fair number of people actually pay up even though the majority of jQuery plugins are free.

Good luck, but don't forget the humble web-portfolio developer out there that is legitimately wondering if they can justify this purchase (i.e. is it easy to use, can i extend it to use two images, will it work on the browsers my client wants, etc.)

edit, typo fix