Not trying to be rude, but it's important to understand that the bank doesn't care one bit about you either. Their job is simply to get as much money as possible for the property, as much of their potential loss as possible. If they find that they can ultimately sell it for more by simply eventually foreclosing on the homeowner - they will just ignore your offer and wait.
The banks don't care what your valuation of the property is, what your agent's valuation of the property is, or what you "want" to pay or get it for. They are like robots and will reject/ignore your offers until they get what they want. They will take as much time as they want to respond, but demand that you "jump" immediately whenever they demand.
At the end of the day, you will likely get a good deal - if you are willing to swallow your pride throughout the process for the end result. Many people are not willing, nor do they have enough patience, to do this however.
Another important bit of information - the "list price" means nothing in a short sale. You would be wise to have your agent check if there is a "pre-approved" price the bank is willing to accept. Listing agents can list a short sale at any price they wish - but the only thing that matters is what the bank will ultimately accept. This means that you can offer full list price, or even above - and it doesn't mean that the bank is going to automatically take it.
The bottom line - Banks aren't stupid and can afford to hold these properties until they get what they want. They are not going to give you a "steal". Do your homework and make an offer that is going to reflect the market value in the area - or you will be in for a long, maddeningly frustrating ordeal that will end in the property selling to someone else.