In the real estate world a letter of intention to buy is most often the first step a buyer takes when they have found the home that they wish to purchase. While it is a non_binding contract, once the letter has been received the seller will usually take the home off the market and stop considering other offers. For those who have never written one a letter of intent sample can be found at several websites on the Internet along with instructions covered how to fill it out effectively. You will find many different versions making it easy to find one that suits your specific needs.
Finally, there is no such thing as an oral offer! An oral offer is really just a test by the buyer to see what the seller will or will not accept. Make the buyer put the terms of the offer in writing as an intermediate step toward securing a legitimate offer.