What's the correct URL for calling the getCategories method? I tried
http://open.api.ebay.com/shopping?callname=GetCategories&responseencoding=XML&appid=" + APPID + "&levelLimit=1&detailLevel=ReturnAll&siteid=0&version=525 but it returns Server returned HTTP response code: 503 The same happens when I try replacing "shopping" with "search" or "trading". The same happens with detail level and without. Can anyone please help with this? I'm spending so many hours on finding the simplest API calls that could be so much better documented. So depressing.
You can't build a GET request to the Trading API (and you method/call you're seeking), but you can do so to the Shopping API, Finding API, and others. There are different endpoints (eBay hostnames and URL prefixes) for the different API families. You really need to read the docs to learn how to go about this. I recommend using GetCategoryInfo in the Shopping API:
http://developer.ebay.com/DevZone/shopping/docs/CallRef/GetCategoryInfo.html You'll also see links at the top of that page to take you to tutorials, call basics, etc, for Shopping API. At the bottom of the page is a sample URL for a basic call to GCI. You shouldn't need to spend all that time trying to figure out the URL structure by trial and error. It's in the docs.