Joomla! has a built in search engine that is expandable to handle any data that the server can access. Using that functionality we have created several search plug-ins for Item search, Category search and more.
This allows visitors to a SimpleWeb site to use one search box and they can search any content on the site, the items you have for sale and the categories your items are in.
And since the search plug-ins are standard Joomla! extensions they will work with other components or modules you may wish to add to your site. The team that developed the Brenner-Fielder site found that the Universal AJAX Live Search module allowed them to have automatic drop down search menus based on what users had already typed. They installed this module and got it working without any help from SimpleWeb staff.