TomKabinet.nl is a marketplace for buying and selling used ebooks similar to the ReDigi service here in the US. The Tom Kabinet website doesn't sell the ebooks directly but instead acts as an facilitator between the buyer and seller, handling both the payment processing (for a fee) and the transfer of the ebook file.
Unlike Redigi, which requires buyers and sellers install an app so Redigi can ensure that a music file is deleted when sold, Tom Kabinet would appear to operate on the honor system with sellers, trusting them to remove the file from their computers. But the service is not quite so trusting with buyers, the FAQ mentions digital watermarks, suggesting that Tom Kabinet adds identifying details to the ebook files during the transfer process.
The current selection in the marketplace is still limited, and is mainly focused on Dutch titles. All of the ebooks are in Epub, and aside from the digital watermarks the ebooks are DRM-free. Due to technical and legal complications, Tom Kabinet has no plans to handle DRMed ebooks from iBooks, Google Play, or other sites, and Kindle ebooks are not supported - DRM-free or no.
So is this legal?
I can't answer that question yet in Europe, but reselling ebooks in the US is almost certainly legal. Thanks to the nuances of the Capital Records v ReDigi decision, it is arguably legal in the US to resell an ebook so long as you sell the medium it resides on. ReDigi continues to act as a used music marketplace under that theory (they even have a patent on it), and last year they announced plans to launch a used ebook marketplace. Nothing seems to have come of that plan, though.
And as for Europe, Tom Kabinet argues that the resale of ebooks is legal under recent rulings from the European Court of Justice. That might be true, but it's no guarantee that Tom Kabinet will win in the inevitable court case.