Rates for insurance are determined by your age and zip code of residence.  No matter what site you go through, including the carriers themselves, the rate for a particular plan will be the same, You are not shopping for a car.

Open Enrollment for Jan 1, 2018 begins November 1st and ends Dec. 15.  If you have lost your coverage due to change of residence, loss of job, spouse losing coverage, then you may apply during a "Special Enrollment" period.