The "Market rate" shown on the credit selling pages represents the average rate that recent credit sales have occurred at. Throughout the day, the rate fluctuates up and down, so this 24-hour average provides a stable approximation of what rate credits are selling at.
It's the recommended rate if you want to sell your credits within a reasonable amount of time. Entering a lower credit per dollar amount (worse deal for credit buyers) than the market rate might take longer to sell, or be less likely to result in a sale. Conversely, entering in a higher credit per dollar amount (offering more credits per dollar) might sell more quickly, but will result in a worse payout for the credit seller.