If you leave your job for any reason and lose your job-based insurance, you can buy a Marketplace plan. Losing job-based coverage, even if you quit or get fired, qualifies you for a Special Enrollment Period. This means you can buy insurance outside the yearly Open Enrollment Period. Your coverage can start the first day of the month after you enroll.
You can also buy a Marketplace plan to provide coverage until your new job-based insurance starts. Until then, you can qualify for savings on a Marketplace plan based on your income. Once you enroll in the new job-based insurance you can keep the Marketplace plan, but you’d have to pay full price. You can end your Marketplace plan any time without penalty.