They're called sellers because they want to sell their home, and presumably get some money now for it. Now, some folks may have an assumable mortgage, but to take over their payments you'd need to qualify through the lender just like if you were getting your own mortgage.
Like the others said, anything is possible but you're probably going to have a tough time finding the situation you are looking for
UNDER THESE CIRCUMSTANCES YOU WOULD PROBABLY BE BETTER OFF SHOPPING AROUND FOR A GOOD MORTGAGE PERSON WHO COULD DIRECT YOU TO A FHA, MISHDA OR OTHER ZERO DOWN LOAN POSSIBILITY
Essentially, you are asking the owner to overlook these realities.