I just read through this thread and found it a great read, thanks for posting.
It seems like you had a very useful experience in trying out this off line niche.
p.s. I don't need to build a site for the Taxi cab niche since I get regular wrong numbers asking for a taxi since my home number is the same minus 1 digit for a local taxi firm. LOL
I wasn't sure why you were optimizing for the dutch version of Antwerp though. Unless you found that many dutch people are resident or tourists in that area.
Anyway, congratulations on selling the site.
The majority of the Antwerp population speaks Dutch, the official language of the southern part of Belgium is French.
I had a few pages in English which attracted few visitors. I thought that travelers would look up taxi phone numbers before arriving in Antwerp.