Unfortunately, not all tickets listed will be sold. If you want to increase the chances of your tickets being sold, make sure you price them as competitively as you can afford to.
Ticket prices are determined more by the market than by face value. With unpopular events, it is sometimes even necessary to price them under face value in order to ensure a sale.
Several factors can influence how much you want to charge for a ticket.
You may want to see if there are other comparable tickets available for sale and see how those are listed. This will help you price your tickets competitively.
Some sellers have unrealistic expectations and will set a price far higher than market value which often results in the lack of a sale.
Note that all fixed priced listings currently active on viagogo have NOT sold and are merely the seller's "asking" price. Listings that are priced reasonably or below market value will sell quickly.
Keep the service fee taxes in mind when setting your price.
Make sure that you understand any regulations from the event organiser that might limit the amount for which you can sell tickets.
If you set a price and later change your mind, you can always go back and change it under "My Account" which is always available from the top of your browser window, as long as they have not been purchased.