I am normally a happy optomist on Trulia but on this question I think I need to take a little different slant. First things first, you will need to get a realtor to help you out here. The process you are about to undertake is way too difficult to do without the help of a professional.
That said, the reality of what you are proposing is not terribly likely. The vast majority of the loans in today's mortgage market are not assumable and as such the seller would have no ability to assign it to you. Another note about sellers in bankruptcy, they are in a terrible financial mess and have with all of the easy money that flowed into the mortgage markets in the last few years, they undoubtedly have a mortgage larger than what the house is worth, so 70 cents on the dollar may not be as good a deal as you think. You would probably be better off finding a house that is on he market but simply listed below market value.
Lets pretend you do find the right house and seller who is willing to let you negotiate with the bank. You would now be working on a short sale as Larry pointed out. These are very common today, but banks aren't likely to just throw 30 cents on the dollar out the window you are going to really need to be able to build a strong case for what you are offering.
There are deals out there, don't get too focused on one particular (and in this case unlikely) scenario and miss the other really good deals that don't fit your mold. I'm sorry to deflate your hopes, but I hope that helps.
I agree that it's a bit much to try to do this without someone working on it that knows short sales and what to do. I can recommend someone for you if you are interested. Good luck with it.
Hope this helps