To get your current inventory listing on amazon do the following
Step 1. Run RequestReport with ReportType set to _GET_MERCHANT_LISTINGS_DATA_
$request = new MarketplaceWebService_Model_RequestReportRequest();
Step 2: Run GetReportList
Step 3: Using the response from Step 2, match the ReportRequestIDs from 1 and 2 and extract the corresponding ReportID
Step 4: Use GetReport to retrieve the data for that specific ReportID which will include a flat file listing of all inventory you have currently loaded in to Amazon marketplace.