When you buy first, you’ll know your new address, the exact price of your new home, the date on which you’ll become owner and the transaction date. You’ll be able to consider these things when you receive a promise to purchase for your current home.
You can also make a promise to purchase that is conditional on the sale of your home. In this case, the owner of the property you’re interested in might receive another offer without any conditions. In order to give yours priority, the owner will ask you to withdraw your condition. You will therefore have to decide whether you’d rather buy that property before selling your existing home, or withdraw your offer.
One way or another, it’s a good idea to plan a temporary source of financing in case your current home isn’t sold when you sign the purchase contract for your new home.