The "search in categories" switch at Admin Panel -> Settings -> Switches enables an easy way for visitors to search for listings in the category they're viewing. An area appears at the bottom of the category in which they can type their search term, select the category to search in (defaults to current) and then perform their search.

This is just a simple search. To do advanced searches in a category, they can go to the advanced search page and select desired categories on the right side of the search listings section.