code: public T marketplace_search(CharSequence category, CharSequence subCategory, CharSequence query)
      throws FacebookException, IOException {
  
      ArrayList<Pair<String, CharSequence>> params =
        new ArrayList<Pair<String, CharSequence>>(FacebookMethod.MARKETPLACE_SEARCH.numParams());
      if (null != category && !"".equals(category)) {
        params.add(new Pair<String, CharSequence>("category", category));
        if (null != subCategory && !"".equals(subCategory)) {
          params.add(new Pair<String, CharSequence>("subcategory", subCategory));
        }
      }
      if (null != query && !"".equals(query)) {
        params.add(new Pair<String, CharSequence>("query", category));
      }
  
      return this.callMethod(FacebookMethod.MARKETPLACE_SEARCH, params);
    }
docs: Search for marketplace listings, optionally by category, subcategory, and/or query string.
  @param category the category of listings desired (optional except if subcategory is provided)
  @param subCategory the subcategory of listings desired (optional)
  @param query a query string (optional)
  @return a T of marketplace listings
  @see <a href="http://wiki.developers.facebook.com/index.php/Marketplace.search">
  Developers Wiki: marketplace.search</a>