The seller must not receive any sale proceeds for themselves.
If there is a junior lien holder, the discounts can be substantial, sometimes as high as 90% or more. Question two is the primary determinant here. If the senior lender forecloses the junior may get nothing so they may take a deep discount to get something out of the property.
Short sale sellers need to be careful because there is no free lunch. The seller may end up with taxable income in the amount of the debt that is forgiven. Except for certain conditions pursuant to the Mortgage Forgiveness Debt Relief Act of 2007, be aware the I.R.S. could consider debt forgiveness as income, and there is no guarantee that a lender who accepts a short sale will not legally pursue a borrower for the difference between the amount owed and the amount paid. In some states, this amount is known as a deficiency. A lawyer can determine whether your loan qualifies for a deficiency judgment or claim.
The seller may also end up with adverse entries in their credit history. Any property owner considering a short sale needs to seek the advice of competent legal and tax advisers before entering into the transaction.
I would advise anyone facing foreclosure to discuss their situation with an experienced Realtor. Short Sales are not a part of real estate basic training but there are a number of educational seminars a Realtor can take to get up to speed. Lenders will pay a reasonable selling commission so Realtors have an incentive to get involved in Short Sale situations.