Agreed.
Besides, would the owner know that you plan on using the ferrets as feeders? I'd be pissed if I found out that the pet I thought was going to a loving home was actually used as snake food. Don't get me wrong- I know that snakes need to eat and I have no problem feeding my ball python her f/t rats. But I would never go on craigslist and "adopt" rats just to turn around and feed them to my snake. I think that is unfair to the previous owner. If you are going to go through with it I think you should be clear about your intentions.