Webcartoonist Ryan Estrada (Plagued, Aki Alliance) has a tutorial complete with widget on how to make an ebook out of a webcomic, and then how to sell it. It really is about as simple as falling off a log, and there is no downside:
So your comic is free online. Why would anyone want to buy a pdf instead of just reading it free? What if you make it and nobody buys it? First of all, waaah. That took like 12 1/2 seconds, total. If even one reader wants an ebook of your work, it’s worth the time investment. And let me tell you something: I buy PDFs all the time. I buy PDFs of comics available in their entirety free online all the time.
It’s true: an ebook can be easily downloaded on a tablet or other device, sold in a variety of storefronts, and otherwise monetized. And making a ebook is simplicity itself.
You can upload the book to ComiXology’s Submit portal, and probably sell it through Amazon or right on your own website. Estrada recommends a “pay what you want” model, but you can try charging a little, too.
I’m guessing that most veteran webcartoonists know all this—successful webcartoonist are like those Thai women who help out William Holden in the Bridge on the River Kwai. They don’t talk about it, they just load shells into bazookas, drag heroes to safety, and do whatever has to be done. They are fast and efficient. But for non-veterans, this is all good advice.