There currently is not a way to set at default listing radius, but this is a good suggestion.

To select more than one category, but not all, hold down the Ctrl key and click on those categories you wish to search. If you click cars on accident, keeping holding the Ctrl key and click on cars again to deselect it. You would use Shift if you wanted to select multiple categories, but did not want to click on everyone one of the. For instance, if you want to browse airplanes, boats, cars and engines. Click on airplanes, press and hold the shift key then click on engines. Everything between engines and airplanes is selected in one go. If you wanted to exclude cars after doing this, press the Ctrl key and click on cars to deselect it, then perform your search.