The only thing I can think of is that the XF staff doesn't get involved in third-party transactions or issues, and may not want to tackle that as it can be a grey area. And that is understandable.
We do the same in our sale/trade area on one of the bigger forums. If buyer and seller have an issue, we are not responsible. However, if we start receiving multiple complaints about a seller, we do take steps to remove their listings and/or forum access until things are rectified. It happens very infrequently, but it does happen nonetheless. A seller can run into a bad run of luck and access removal might be temporary, but more often than not they've received money and are never heard from again. In which case their removal ends up being permanent as we never hear of a resolution.
A good solution for unmaintained resources might be:
- Paid: Disconnecting the paid link button to the developer's site, changing it to a "Contact developer" button which redirects to the direct message system instead.
- Free: Still allow a download, but with a popup warning that the addon is unmaintained and may not be compatible.
- Make it clear in the site policy what happens to neglected addons.
Or to put this another way, I would feel as if I were helping commit fraud if I left up a paid link to a resource where I've had multiple reports of a developer taking money for an extended period and not responding.