For you, sure. But there are a lot of communities running entirely there. Our local farmer's market is preorder only, and is entirely run off Facebook. I would have to skip going there altogether.
And it's not viable to propose any alternative. There is no platform, closed or open, where there are enough people registered to be actually usable, and nobody will make a separate account for this.
You're confusing the worth of the book and its content. A book can be valuable (ie a rare bible print), whereas its content is not (we have all the bible variants copied).
German copyright law has no Fair Use. There are copyright exemptions for private or academic use, but commercial use has no such exemption. Books have special protection even according to § 53 (4) b).
And yes, it's the duplication which is illegal, not the publication of them.
Given how fragile unreliable those processes are, you probably can without doing much damage. Its not like replacing good quality software with something vibe coded.
And it's not viable to propose any alternative. There is no platform, closed or open, where there are enough people registered to be actually usable, and nobody will make a separate account for this.