Hi, Cinnamon, thank you very much for your answer. To your first question: no, I don't. Where could I find them?
(I made a screenshot of what I get on XStreet, but don't know how to post it (as well as I couldn't find any button like "answer" or a field for a new comment on both of your answers. I can only give kudos, accept as solution, email your answers to friends, report inappropriate content etc, but no "answer" :-/ )
Nevertheless, I have a list of the content on the left side on XStreet, and an empty space on the right, saying "These are listed items, that are not connected to an active inventory (stock) and hence can't be delivered".
The communication between the Box and XStreet / Marketplace is definitely failing. But I checked the group, which is the right one, and scripts are enabled on the land anyway. You see, I'm desperate, because so far, I checked everything and still it doesn't work :-(