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I'm trying to use GetCategoryFeatures to return the valid listing durations for a site. However, GTC is missing. What's going on?
Depending on the site, not all sellers will get be able to use the GTC listing duration. For some sites, only store sellers will be able to list GTC items. To figure out what the valid listing durations are, you should use GetCategoryFeatures. However, since GTC is not valid for all sellers, GTC is returned in a separate container called 'StoreOwnerExtendedListingDurations'. You can read more about StoreOwnerExtendedListingDurations here: http://developer.ebay.com/DevZone/XML/docs/Reference/eBay/GetCategoryFeatures.html#Response.Category.StoreOwnerExtendedListingDurations .
It's the responsibility of the application to 'consolidate' these store listing durations with the 'normal' listing durations, when appropriate.