Thanks, Harmonie - I posted in the classifieds too.
I actually just found a secondhand bike shop in Katutura that people might be interested in: It's a non-profit called , where former sex workers train as mechanics to get off the streets. Operates from a converted shipping container in the lot of the Council of Churches of Namibia, at the corner of Mungunda and Florence Nightingale. Apparently they do Cardboard Box tours of Katutura starting there too. I picked up a 15-speed mountain bike for N$750, or $89 U.S.