You are required to syncronise the eBay categories, your shop categories (if you have an eBay shop) and also your localised settings for your marketplace (such as shipping options, payment options etc).
If you have errors when listing a product saying that the category is invalid, the payment option is invalid or shipping service is invalid - you should click through each one of these options to update the values stored in your store.
Updates can take about a minute and confirmation message will display when it is complete. You should not navigate away from the page until you get the message.
If your loading icon continues to spin, you should check your OpenCart error log (located under system/logs/)
Some users may see a "MySQL server has gone away" message in the error log and the loading icon continues to spin. This is because the store is taking a while to process the data is it getting from the API. To fix this issue contact your host and ask them to change the MySQL setting "wait_timeout" to something over 5 minutes (or 300 seconds) - the default is 30 seconds and is too low for most modern applications. Any good host will do this for you, although some cheaper less reputable hosts may not. In this case you should consider a host that will accommodate changing settings based on your needs.