I spent an eight year stretch successfully building spec. houses for a living. The ONLY time I really got a good screwing was when a realtor talked me into signing a "contingent on the sale of the buyer's existing home" contract. IMHO, I wouldn't waste the paper to write anything. If they are not buyers until they sell their current shack, you need to move on to qualified buyers, if they turn into one great, if not, no worries, no stress.