Yes, if you are not covered by Medicare, an insurer can sell you a Marketplace plan. But because you are eligible for premium-free Medicare Part A, you are not eligible to receive the premium tax credit to help reduce the cost of a Marketplace policy, even if you would qualify based on your income.
Also keep in mind that if you sign up for a Marketplace plan, rather than enroll in Medicare Part B when you are first eligible to do so, and then later you decide to sign up for Medicare, you may be required to pay a penalty for delaying enrollment in Medicare Part B. Your monthly Part B premium may go up 10% for each year that you could have had Part B but didn’t. You may also owe a late enrollment penalty for Part D drug coverage, which is equal to 1% of the national average premium amount for every month you didn’t have coverage as good as the standard Part D benefit.