The pictures didn't load when I looked at the ebay page and the text of the ad does not say which brand/model is it. Generally ethernet cards work except for power issues. Socket, for instance, makes a 10/100 card and a low power one. The former doesn't work because the Zaurus can't provide enough power to it, while latter works fine. Don't know anything about the seller and I guess the price is OK. But I would try to find out which brand and model it is before buying.
Felipe