You and others might enjoy this article, written by Julia Wild in German. She describes the trade, and includes some family stories. It has some great pictures too. The family has documented things very well, and I could have spent many more hours sifting through all their pictures and stock.
I am pretty sure that between her and her father I have spoken the most knowledgable people around when it comes to this agate trade.

The link goes to a direct download of a German magazine, which includes the article. The article was made from the lecture that Julia gave at the opening of a 2010 exhibit in the museum on 'African Money'.