When I first started using the Finding API a couple years back, I wanted to retrieve auctions via findItemsAdvanced. Most everything worked fine, except I had an issue with eBay Motors.
I noticed that when I used the &categoryId= filter to search category #6000 (all of Motors), I got an Error #3 "Invalid Category ID" error. I was sure to use the proper GlobalID of EBAY-MOTOR, and I was disappointed to infer that eBay wasn't allowing us to search through the entirety of the US Motors site. :(
Solution/Workaround! I found that if I used EBAY-US instead of EBAY-MOTOR for my cat#6000 search, the Finding API kindly translated that request to US Motors behind the scenes, and did indeed allow me to search all of Motors vicariously. Nice.
Bummer though... Fast-forward 2+ years: Within the past week, this courtesy mapping/translation has partially failed, and I can no longer search the entirety of cat #6000, whether I use EBAY-MOTOR or EBAY-US. :(
There's an exception: it works fine if I also specify a keyword. In other words, using category 6000 by itself (no keyword) generats an "Invalid Category ID" error -- but with a keyword, results are successfully returned (suddenly the category# is valid).
So, just a simple request to eBay here... Is this new limitation here to stay? Or perhaps, hopefully, just a temporary/fixable bug?
(If anyone's wondering how constructive it could be to search all of cat=6000 without specifying a keyword, it's because using a combination of filters and/or sort-orders can yield some helpful results.)
Edit the above link to EBAY-MOTOR for comparison. And/or add &keywords=Harley to see some search results returned.
I get the feeling that a cat=6000 search without keywords working fine via EBAY-US was a loophole that they just patched up -- a bug to them, but a feature to me. ...I hope I'm wrong.