My thinking is that if the previous owner's account details are still on the device and that account is still active, that every time that you wifi sync you're going to get a copy of every purchase that person makes.
It may also mean that if you buy a book through the device, that the previous owner would be charged for it.
So that suggests to me that you would be knowingly committing fraud.
I would delete the previous owners account info post haste ..... it's just not worth it.