Are you seeing the following?: "Sorry, these tickets are currently being booked by another buyer. Try again shortly or check the tour for more tickets"
This means that somebody else has got to the listing before you and is going through the purchase process. Whilst they do this, the tickets are held for 10 minutes so that nobody else can purchase them. If the user completes the transaction within that timeframe, the tickets will be removed from the site. If they do not, they will become available for sale again.