By default, all products in a product type will sync or not sync with TCGPlayer. There is a way to prevent certain products from syncing, though.
To do this, you will need to edit the product type on your Inventory > Types page. Under the "Condition" section of the product type, click on the "New Condition" button just under the condition list to add a new value. You can name this value whatever you want. For example, if you wanted to name it "Website Exclusive", it would look like this:
What allows products to sync with TCGPlayer is the "Official Catalog" mapping in the product type, below the condition list. So, in this case we are going to make sure to keep that blank:
Now, when you add quantities to this new condition value you've made, those quantities will be exclusive to your website and POS, and won't sync with TCGPlayer.
Note: the Official catalog mapping is also required for listing to Amazon, so this will prevent these conditions from being able to be posted to Amazon as well. There is not currently a way to hide the product from TCGPlayer but still make it available on Amazon.