You can access and view your open offers from the Selling tab of the My Items menu. Once you review the offer, you can accept the offer and the person who made the offer will be notified and prompted to buy the item. Or, if the offer is too low, simply decline the offer. You may like to also send a message via Chat such as, "Thank you, but I would really like to get at least $50." Either way, aim to respond to the offer within 24 hours.