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I am sending the GalleryURL tag during the RelistItem call, however the Gallery image is not getting updated.
<GalleryURL>New Picture URL</GalleryURL>
The intent of eBay's selection rules is to have the gallery thumbnail match the picture that appears at the top of the listing's page (which is specified by the first PictureDetails.PictureURL) whenever possible, as long as this picture is valid. As an example, if both PictureURL and GalleryURL have been included in the AddItem request, eBay uses the picture referenced by PictureURL as the gallery thumbnail (copying this URL into GalleryURL). As a result, both the gallery thumbnail in the search results and the picture at the top of the listing's page now match, providing a more consistent experience for buyers when browsing from search results to listing pages. If only the PictureURL is specified in the AddItem request, then the value gets copied over to the GalleryURL and vice-a-versa.
Now in the Relist request the GalleryURL was sent, however the PictureURL was present from the original listing. Since both were present, the GalleryURL was ignored and the PictureURL details were passed over to the GalleryURL. Thus the image sent in via RelistItem did not appear as the Gallery image
As a best practice for specifying pictures we suggest users to use PictureURL and omit galleryURL entirely (for the sites with Free Gallery) when making the RelistItem call.
This also holds true for revising the gallery image for the item via the API.
Here is a detailed description about the best practices and expected behaviors.