Yes, there are rules associated with selling alcohol on eBay - see the ebay alcohol policy for more info.
You will need to add to every listing a disclaimer supporting the responsible service of alcohol or whatever the regulations are for the states you sell into. e.g We support the Responsible Service of Alcohol. New South Wales: Liquor Act 2007. It is against the law to sell or supply alcohol to, or to obtain alcohol on behalf of, a person under the age of 18 years.
To add this to every product listing if you are using the Omnivore app go to 'Settings - Feeds - Description Template' and add the disclaimer here.
To add this to every product listing if you are using the eBay Link app go to 'Settings - Feeds - General' then and add the disclaimer to the Description Template following the instructions here.
It will be applied to every product that you list on eBay. If you only need it on a specific product, go to 'Inventory - All products' Find the product then on the full product details page click edit. Scroll down to 'detailed description' and enter the disclaimer here.