There are many reasons why banks accept short sales. The main reason is because the payments are late and the homeowners can prove that they can no longer afford the property.
-The property does not have to be in foreclosure for the bank to accept a short sale. Some banks require the foreclosure notice to be served, while others will accept a short sale when just a few payments are late.
There is no specific number of payments that must be delinquent. Even one payment is enough at times. Often homeowners will call you when they are not yet in default, but cannot make any more payments. In this case, contact the bank, let them know that the homeowners will not be making anymore payments, and open negotiations for a short sale before the payments are even late.