It could be what they said below, or a HUD home where you need to provide you SS# in order to bid. Ask your real estate agent to check. It might be completely legit, or it can be something else. I would be very careful with giving my SS# to a stranger.
Bank of America short sales (not REOs) require the buyer's social security number. This is required by Bank of America to ensure that the buyer and seller are not related. If you are not comfortable sharing this information, as is certainly your right, I probably would not purchase a Bank of America short sale. Given the recent credit card security breaches and explosion of identity fraud, I can understand your hesitation.
I have an idea... ask the listing agent to use his social instead! :)