title: eBay API - filter items by store/custom categories
body: <p>My scenario: I have a client with an eBay store with custom categories. On their website, they would like to list the items they have for sale. They would like this list to be filterable by custom category.</p>
  
  <p>I've been searching for a couple days now, and I'm still a bit unsure about the best way to proceed.</p>
  
  <p>From what I can gather, the only way to get a list of categories defined in a store is through the GetStore call in the Trading API. This seems to be supported, though I need to get a user token from my client's eBay account. I will assume this works.</p>
  
  <p>I do not, however, see any method of filtering items based on custom categories. I've looked through the documentation of both the Trading and Finding APIs, but all I see are filters on pre-defined eBay categories.</p>
  
  <p>Is it possible to filter on custom/store categories?</p>