On listings where the seller is allowing prospective buyers to make offers, you will see a “make an offer'' button below the “buy” button. When you click this button you will be able to make an offer on the selling price of the ticket. You will not be able to offer less than 30% of the original selling price of the listing.
Please note that making an offer does not hold the tickets. They will still be available for purchase by other fans until your offer has been accepted and you have purchased the tickets.