I think this has been raised before - but worth flagging up again...
When a selling category has no sub-categories, the indication from the Buy tab (e.g. http://uk.ebid.net/buy/business-office-industrial/) is that there are no items listed in that category:
Would it not be better to indicate the total number of listings next to the main category? - as below:
This would give the browser some incentive to seek further, rather than getting the impression there is nothing beyond that point.