The bank will want you to work with an agent who is very familiar with the process. Short sales already take long enough, if all sellers started negotiating their own short sales the process would clog up the banks' systems.
That reason above is not really the best answer I could possibly give you.
Really, you do NOT want to negotiate your own short sale. All the bank wants to do is try to get more money out of you any way they can & you as the owner, have no idea what you Must tell them, what you don't Have to tell them & what you Need to tell them about your current income & assets.
A Realtor who is a short sale expert, their job is to get you in & out of your short sale with the least amount of impact on your credit, making sure you don't have a tax liability (if it's your primary residence) , make sure you're not set up to get hit with any future deficiency judgments & make sure you're not coming out of pocket to close.
I came across your question as I was looking for a particular rental scam in Southern California, yours just came up as a "most recent question", anyway, if you'd like I'd be happy to find a REAL Short sale expert in your area, I know what questions to ask to find the right person for the job, it is so important you use someone who has a good track record of closing short sales, but who also has a 95%+ success rate in closing them. For example, I know an agent in my area who has closed 34 short sales, although that number is high, it actually represents a 50% success rate in short sale closings (not good)
Email me directly if you'd like, I don't look back on this same Trulia thread for answers after mine.
Realtor Since 1996
Main Street Realtors
Short Sale Expert