It's worth asking the realtor where they came up with a $60,000 value from. I'm not sure about the full process there but when I've worked with Short Sales here the bank doesn't agree to a price until they have sent their appraiser out to appraise the home. Then they will counter with the appraisal value they come up with.
Determining the value of home can be tricky but in the end the Bank is out to get the most they can for the home - they are the ones loosing money on the deal. It's a tricky situation that can last for a while and it boils down to how much is the bank loosing.
If the BPO came in at $99,500 there must be comparable sales to support that price. Does your agent have comps that support his value of $60,000? If so, he/she might want to point them out to the listing agent to substantiate your offer of $70,000.
Your realtor told you $60k was the current value, why would you offer $70K? This is not a used car, where you might pay a couple hundred more because you like the color. Even if they accepted 70K, what would you do when the appraisal comes in at 60k or 62k? Your lender will only lend on APPRAISED value. I think your time would be better spent finding another property.