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Why is the IncludeItemCompatibilityList null in my GetItem response?
If you are seeing that the IncludeItemCompatibilityList is not being returned, this may be working as expected. Some listings are catalog fitment listings , for which we don't emit the fitment information in GetItem. Only for manual fitment listings we provide these details in this API. The right way to be retrieving catalog fitment data is with the eBay product id (ePId). You can do this by using the Product Service API to fetch the fitment information.
The current process to get catalog fitment data from listings you need to use the getProductCompatibilitiesRequest API, http://developer.ebay.com/DevZone/product/CallRef/getProductCompatibilities.html
That above will only work for specific items. If you need to find combinations based on an identifier, use the Master Vehicle Lists for Car and Truck categories will be updated monthly ( usually towards the last week of the month). At this time there is no subscription feature however if the seller is a large seller, they would receive notifications from the fitment council. The Latest version of the master lists ( vehicle, Motorcycle, etc) are posted on the download link, that can be bookmarked: http://pages.ebay.com/motors/compatibility/download.html