Please follow the below mentioned instructions in order to find you eBay Store Category ID:

Step 1: Login to your eBay store

Step 2: Click the Manage My Shop link from within My eBay (on the left)

Step 3: You can then click Shop Categories, where you will see a list of your Shop Categories

Step 4: If you scroll down and click All Categories at the bottom right, it will show you a list of all of your categories and sub-categories, and their associated category number.

Remember you can only list in LEAF categories
( category that has no child categories)

Hope these instructions help. If not, then please feel free to contact SellerCahmp support and we'll be happy to assist.