You decide your sell price for your credits (credits per dollar), but as with any marketplace, pricing your credits correctly will determine how quickly and whether or not your credits sell. Once you have access to the feature, the credit selling page will contain a couple graphs which show recent sell rates and current open sell orders to help you determine how to set your price. Remember that the best priced credits (most credits per dollar) always sell first.
If you find that your credits are not selling, you can cancel your open sell orders at any time and place new orders with adjusted rates to help them sell faster. When you cancel open orders, unsold credits will be refunded to your account.