Security for sellers - notarized preliminary contract?

I have already experienced it during the property search. A house/property has several interested parties, some increase the price to get it. For most agents, money is the most important thing. It is unfair, but whoever is faster and offers more can acquire it. The second thing is humor and sympathy, very important. The second is always needed and can be sufficient, if not, then the first one, money, is needed as well.