What would happen if I never buy Craft license and continue working in Craft CMS Pro trial ? Would the CMS be locked after some day or a error will show up on my website frontend ?
They have answered this on the CraftCMS site: https://craftcms.com/support/license-enforcement
Each time Craft phones home to check for updates, we check the incoming license to see which edition it should be running. That “licensed edition” is included in the response, which Craft compares against what is actually installed. If there’s a discrepancy, the Control Panel will display a modal window that provides options for correcting the situation.
You can locally, but launching the project into the wild, you will need a pro license.