Whilst you have not reached your quantity limit, you have reached the dollar amount limit. The 423 items listed multiplied by their prices and quantities have reached $15,000.

Don't take it personal, it is an eBay security feature to prevent someone hijacking your account and listing Thousands of dollars of high end equipment as part of a scam. Ebay does not know if this is occurring and want to make sure that you are who you say you are in trying to exceed your limit (which you can ask them to increase) 

If you don't want to go through the process but still want to list more, reduce your quantities so you free up the dollar value ie if you have listed item "z" at $50 qty=10, change it to qty=3 , and you free up $50x7=$350 so you can list new item "y" at $20 qty=1 and more items up to the $350 you free up. As you sell "z", you ca then adjust qty from (now 2) back to 3