I'm surprised no one has answered yet!
I assume there arre many reasons. Some people don't speak English well enough - or think they don't - and so prefer to post in German. Also, the fact that the website is displayed in German for most of them might make them think there is a strong enough German presence, making it okay to post in German too. I have noticed that many just like to keep to their own group of countrymen/women too so there is seldom a need to venture into unfamiliar foreign territory. The primary objective seems to be to sell within Germany rather than internationally. I wonder if anyone else has any better ideas.
It all sounds like I speak as an outsider, but I am actually German. if I didn't live in the UK as I do now, I would still post in English, of course. I would be aiming to sell wherever buyers are and that requires me to use the language most people would be familiar with, i.e. English.