Performs the Listing Method: Retrieve the details of a single listing
using the "query" string provided - should be the "Listings/{listing_id}.xml" part of the url. It shouldn't include "http://api.trademe.co.nz/v1/".
public ListedItemDetail ListingDetail(string query) { if (_listing == null) { _listing = new ListingMethods(_connection); } return _listing.ListingDetail(query); }