Offering on short sales is "proceed at your own risk" as far as I am concerned, as MANY listing agents haven't done a single one.
I'd also advise you ask your agent to ask the other agent how many they've done and if they use a negotiator or not.
Best of luck. I can't tell you how many short sale purchasing nightmares we've gone through just like this...most of my buyers avoid short sales like the plague!
Although you didn't mention how close the price/terms of the "accepted" offer were to yours, I'll assume that they were off enough for you to be unwilling to accept them as-is. Nevertheless, that response might have been a blessing in disguise for you. You should have your agent inquire about the BPO. If the BPO is older than 30-60 days, then have your agent to request another one. If the BPO is relatively close to your offer price, then you might be able to negotiate to get the bank to lower the price, offer some concessions, or at least meet you half-way.