You can search using the search bar or the listing categories.
Search Bar
When you first the marketplace you will see search bar above the map. You can choose the listing type (share/swap/sell/wanted) then enter keywords.
Map
On the map you can zoom in and out using the – and + buttons to narrow down your search.
Listing Categories
You can also choose the category you are interested in and see what listings are available and where they are.