Konstantin selling something on eBay/Marktplaats is a common thing and works fine. There's a lot of scamming taking place on these platforms so from time to time you may bump into extra checks causing your transactions to be inspected by compliance officers. This happens automatically, and when no fraud is there (e.g. you can show it's a legit transaction) then it will be approved + finalised within a day/few hours typically.
The exact rules can not be shared for security reasons (and are only known by bunq in the first place). This is also why some users complain about getting "no reason" for their account being closed; bunq is not allowed to share this by law typically. I only know that they look at a massive amount of parameters to identify potential cases of fraud and other suspicious transactions. Again, when you use your account like you would use any other bank account you will have no problems typically. I've been using bunq for quite some time now and never experienced anything like this (only a single delayed transfer because of Marktplaats).
On the internet you will typically find people complaining, people do not really often write "yeah, everything is working fine" on these forums. So this gives a scewed view on what's actually happening. 🙂