title: Correct Amazon MWS flow for item with existing ASIN
body: <p>I'm getting started with Amazon MWS and I can't seem to see any real information on the correct flow for listing an item as an existing ASIN. Let's say for example I am selling a "Vulli Sophie the Giraffe Teether". I do an initial lookup using "listMatchingProducts" and find that my item already exists with the ASIN "B000IDSLOG". What is the next stage in the process?. All the documentation talks about the fact that the product feed is intended to match our SKU to the Amazon ASIN but i've not seen any definitive information to suggest how this actually works - especially in the scenario where you already know the ASIN you wish to use.</p>
  
  <p>Ideally i'm interested in seeing the correct flow for each scenario (existing product for search found/not found) in terms of what API calls should be made in what order. </p>
  
  <p>Thanks</p>