Q:

Ebay API linking relisted items by identifying a common field or key column

I'm new to the Ebay API and I'm looking for something specific from either the Finding or Trading call responses. Maybe thats the problem, I may be looking in the wrong place. 
What I need to do pull items that have been relisted multiple times, by Ebay changes the item number, so tracking an item is very difficult from the front end. With over 1000 listings every week, this can get impossible to track. There are common fields like title but you can imagine that if some change the title along the way, link for the product would be lost and the report would be incorrect.
Is there a common key or any link between the original listed item and the relisted item? Here are the references I'm using:
http://developer.ebay.com/devzone/xml/docs/reference/ebay/index.html

http://developer.ebay.com/devzone/xml/docs/reference/ebay/RelistItem.html

A:

Typically people will use the Custom Label field to identify a product.  I believe this becomes available when you are using Selling Manager Pro on eBay.  Basically when you start using SMP (Selling Manager Pro) it gives you an inventory system built into eBay.  You identify a product with the custom label, and then you list using the product listing template.  This enables the user to reference multiple listings based on a common identifier, many times, an internal company SKU.
The listing process if you are doing it programatically, changes a bit as instead of using simply an 'AddItem' call, you need to create the product, create the listing template and then add the listing from the template.
Additionally, some (many possibly) shipping programs use this custom label field to tie into your local inventory system (I know ShipWorks uses this).

