To avoid users placing low offers on your ad, you have the option to place a minimum offer. This is the minimum amount you are willing to consider on your ad.
When placing/editing your ad, in the price and payment box you will see the option to 'set a minimum offer'. Select this and a box will appear below for you to enter your minimum offer.
Although this will not be displayed on the ad, it will still be public. If a user tries to place an offer below your minimum offer they will see this pop up:
If you wish to have an non- negotiable asking price, enter the same asking price as your minimum offer. Your ad will appear as non-negotiable and buyers will not be able to place any offers other than that price.