David Revoy creates the series entirely with free software, such as Krita and Inkscape, making the Krita source files for each image available for download. All artwork is made available under the Creative Commons Attribution 4.0 license. He explicitly encourages fanart and other derivative works.

They sell books which is kind of a tradeoff, but its more like a way to support the artist. Because the digital copies are free, anyone can read or print the book by themselves. On their shop where they sell their books they give you 3 options – soft cover, hard cover, or download it as a PDF. You can see they sell it as a PDF while you can already get that for free, it shows that its more like a donation to support the artist.

しかし、Tシャツやステッカーなど、他のものも販売しているので、3ブロックしか与えません。

彼らのデジタル作品はトレードフリーのようですが、もちろんTシャツ、ステッカー、漫画本などを販売しています。ただし、オンラインで漫画を読んだり、アートワークをダウンロードしたりできるので、2ブロック与えます。

I would give 3 blocks, since they indeed offer their comics as trade-free, however they also link to a shop, facebook and twitter page (trade-based networks). And the ad to their shop is quite “in your face”: https://www.directory.trade-free.org/wp-content/uploads/2020/12/Pasted.jpg