When lenders agree to a short sale in real estate, they are accepting less than the total amount due on the loan. Not all lenders will accept short sales and not all sellers or all properties qualify for it. Sometimes it may simply make more financial sense to the lender to foreclose.  However, under the right conditions, a short sale can save lenders many costs associated with the foreclosure process – attorney fees, the eviction process, delays from borrower bankruptcy, damage to the property, costs associated with resale, etc. In a short sale scenario, the lender gets the property back faster and that time advantage helps to mitigate losses. The job of the buyer/investor is to convince the lender that it will fare better by accepting less money now.

General Principles for Buyers:

Lender may not accept your offer, even if the seller does – Lenders aren’t emotionally attached to their properties, so they aren’t as likely to give you “steal.” Many short sales fall through if the Broker Price Opinion comes in too high.

Check the public records – Do your research. Your agent can find out who is in title, whether a foreclosure notice has been filed and how much is owed to the lender(s). If there are two loans, you could have a problem. If a seller owes $160,000 on the first and $40,000 on the second, offering $160,000 leaves nothing for the second. The first will need to give some consideration to the second in order to gain its cooperation.

Homes sell near market value – If a lender believes a better price can be obtained by taking the property back in foreclosure over a short-sale offer, the lender may hold out for a higher price. That price will be close to market value.

Homes sell “As Is” – If a mortgage company agrees to a short sale, it is most likely also paying the closing costs in the transaction. Lenders ask buyers to purchase the home in its present condition. So be sure to reserve the right to have home inspections and ascertain if any defects would rule out your interest in the property.  Lenders typically refuse to pay for:
•    Suggested repairs disclosed on a home inspection.
•    Pest inspections or work necessary to issue a clear pest report.
•    Roof certifications or roof repairs.
•    Home protection plans for the buyer.
•    Deferred maintenance.

Length of Time to Close – Depending on when the Notice of Default was filed, the lender’s back-log of foreclosures and how much paperwork the seller has already submitted, it could take anywhere from two weeks to two months to get a response on a purchase offer from a lender. It can take even longer if more than one lender is involved. Consider giving the lender a deadline, if timing is important for you.

Lenders Can Change Conditions – Some lenders reserve the right to renegotiate the terms of the short sale at the last minute. If the market changes, new laws pass or new information crosses the lender’s desk, the lender can attempt to change the terms of the contract.

Lenders Discount Broker Commission – Agents often end up doing two to three times the work of a conventional transaction and sometimes are paid less. If you have agreed to pay your agent a certain percentage under a buyer broker agreement, you could be liable for the difference between what the lender will pay and what your contract stipulates, if the agent refuses to waive the difference.

Higher Buyer Closing Costs – Sometimes lenders will refuse to pay for standard seller closing costs such as transfer taxes. If you want specific inspections, you will probably pay for them out-of-pocket.

Indefinite time frame – A short sale home closing process takes an indefinite amount of time. The seller’s lender calls the shots, so if you are trying to close escrow concurrently with the sale of your home, it might not happen.

Little Seller Motivation – Since the short sale effect on credit is identical to that of a foreclosure, there is little incentive for a seller to cooperate with a short sale.

Reference: http://homebuying.about.com