My application supports listing in a very wide variety of categories. Is there a way to get recommendations for all categories of a site at once?
Yes that is possible. Here's how you could go about it:
In the Trading API, make a call to GetCategorySpecifics for a particular site (e.g., site ID 0) and set CategorySpecificsFileInfo to true. The response will return a FileReferenceID and TaskReferenceID. These IDs identify the file that contains recommendations for the specified site.
Pass these two IDs as input to the downloadFile call in the File Transfer API. downloadFile returns a standard "Success" response, plus a .zip file.
Extract the GetCategorySpecifics response from the .zip file. The XML file contains all available recommendations for all categories on the site.
Once every day or two, call GetCategorySpecifics again to see if the FileReferenceID and TaskReferenceID have changed. If they have changed, use the new IDs to download the latest version of the file. Note that a different file (with different IDs) is associated with each eBay site.
Sample request and response for GetCategorySpecifics
If you call GetCategoryFeatures, you may detect that custom Item Specifics have been newly enabled in certain categories since you last downloaded recommendations. In this case, if the file IDs returned by GetCategorySpecifics haven't changed, you can call GetCategorySpecifics directly to retrieve any new recommendations for those specific categories.