Most homeowners are unfamiliar with the concept of a short sale, so you may need to give them a crash course on this process.
The lender will not consider a short sale unless there is no equity and the seller can not repay the loan balance. The lender may require the seller to demonstrate that previous attempts to sell the property have failed.
The seller cannot receive any money from the proceeds in a short sale. Sellers may also be responsible for taxes if the lender sends them a form 1099. Explain to the seller that the difference between the loan balance and the amount received in the short sale may be declare as income on their income tax return.