Thanks for posting your question here. While we do not have anything in place that will block you from posting an item manually with a quantity that is greater than the number you have in inventory, you can configure your Auto Relist Profiles to only automatically relist your items if you have over a certain amount left in inventory.
Most sellers who use our Inventory functionality along with Auto Relist Profiles want their items to automatically relist regardless of whether or not they sell until they have a certain quantity left in Inventory, although I certainly realize it is possible you have different needs.
If you would like to set up an Auto Relist Profile that will relist your items automatically in this manner, you will want to select the "Don't Auto-Relist based on items sold" and "At least ___ items are available in Inventory" options in the "I Want to Auto-Relist if" section of the profile.
Since you typically post listings that have a quantity of 20, I would recommend setting up your Auto Relist Profile to automatically relist your items as long as you have more than 20 left in inventory to avoid the risk of potentially selling items you do not have in stock.
There are also a couple other tools available on your Inventory Preferences page that can help you avoid overselling your items called Post Threshold and Stock Threshold which you can set up by mousing over the "My Account" tab within your account and selecting the "Inventory" option in the "Settings" section.
If you enter a numeric value in the "Post Threshold" field, our system will warn you if your stock level reaches or attempts to surpass that threshold during the listing process. If you enter a value in the "Stock Threshold" field, our system will send you an email notification when any of your items reach that stock level.
I hope this helps. If you need any further assistance with your account, please feel free to contact our customer support team by mousing over the "Help" tab within your account and selecting the "File Support Case" option.