Change up what you are offering or what you are willing to accept.
If the price is firm let them know that you will meet the price but you need a bit more in the deal. So if you are going to pay $200 for the ammunition insist on an extra case or another weapon to go with the price. If you are selling you may say you will be willing to sell at the price they are offering if they are going to buy more of the item.
So instead of selling them one dozen eggs at $5 you would sell them 5 dozen at $5 each. Regardless of how you change it up you will need to make sure they understand you are willing and even eager to make the transaction work by looking for alternative methods to make an agreement.